Polish forms components and split out CSS files

[janfaracik]

This PR is the first of several follow ups to #5923, the intention of this PR is to polish the new form components and to also modularise the styling of the components.

• Form components have been polished, icons have been updated and animations have been tweaked to be more consistent

• CodeMirror input has been updated to look the same as textarea

• Component styling has been split out into individual files to make it easier to maintain
• Some small spacing adjustments + headings have been updated to be the same size as .jenkins-app-bar without the need for the additional wrapping

Proposed changelog entries

• Minor form and spacing changes.

Proposed upgrade guidelines

N/A

Submitter checklist

• (If applicable) Jira issue is well described
• Changelog entries and upgrade guidelines are appropriate for the audience affected by the change (users or developer, depending on the change). Examples
  • Fill-in the Proposed changelog entries section only if there are breaking changes or other changes which may require extra steps from users during the upgrade
• Appropriate autotests or explanation to why this change has no tests
• New public classes, fields, and methods are annotated with @Restricted or have @since TODO Javadoc, as appropriate.
• For dependency updates: links to external changelogs and, if possible, full diffs

Desired reviewers

@mention

Maintainer checklist

Before the changes are marked as ready-for-merge:

• There are at least 2 approvals for the pull request and no outstanding requests for change
• Conversations in the pull request are over OR it is explicit that a reviewer does not block the change
• Changelog entries in the PR title and/or Proposed changelog entries are correct
• Proper changelog labels are set so that the changelog can be generated automatically
• If the change needs additional upgrade steps from users, upgrade-guide-needed label is set and there is a Proposed upgrade guidelines section in the PR title. (example)
• If it would make sense to backport the change to LTS, a Jira issue must exist, be a Bug or Improvement, and be labeled as lts-candidate to be considered (see query).

[NotMyFault]

• Amending to my other comment here
  
  Changing the theme for console highlights looks quite nice in dark theme, but a bit odd in other occasions, such as /log/all for example.

• The scripting console's expandable textarea drag button does nothing, though it wasn't expandable in the first place
  
  Maybe move the "Run" button a bit lower, so it doesn't cut off the new text box frame too soon?

• Unless that is a regression from the form components PR, you can't change the size of the textboxes at /editDescription, whether job or Jenkins' description, anymore, if you attempt to drag it smaller or larger.

[fqueiruga]

Code looks good, haven't tried it. This is some great refactoring!

[fqueiruga]

LOVE all of this

[timja]

Only a couple of nits but non blocking,

1. script console looks like it can be resized but it doesn't let you
2. dark theme should probably have the pre-background variable set to increase the contrast on the console log

[NotMyFault]

Only a couple of nits but non blocking,
script console looks like it can be resized but it doesn't let you

Yeah, that actually is a regression from the form components PR that can be addressed separately I guess. Same applies to the job description text box, but Jan already took care of that 👀

The new <pre>s look much better imo, only nit I have are, that links now use the same color as regular text, used to be blue before.

[janfaracik]

Only a couple of nits but non blocking,
script console looks like it can be resized but it doesn't let you

Yeah, that actually is a regression from the form components PR that can be addressed separately I guess. Same applies to the job description text box, but Jan already took care of that 👀

The new \<pre\>s look much better imo, only nit I have are, that links now use the same color as regular text, used to be blue before.

I've fixed the link colour in <pre/> 👍 The resize issue is really weird, works in Safari but not other browsers - will need a good Google to find if others have a similar problem with CodeMirror ...

dark theme should probably have the pre-background variable set to increase the contrast on the console log

I'll update the theme 👍 PR here for that jenkinsci/dark-theme-plugin#169

[timja]

This PR is now ready for merge, after ~24 hours, we will merge it if there's no negative feedback.

Thanks!

[daniel-beck]

Contributed to JENKINS-69549.

[basil]

@janfaracik This change seems intended to fix some particular problem, but it's unclear which problem it was intended to fix and in fact it causes a new problem as described in JENKINS-69549 and #7078. In order to ensure that the fix for that bug doesn't re-expose whatever problem this original change was intended to fix, it would be helpful to know what specifically this original change was intended to fix.