Make branding overridable in signin/login page

[Vlatombe]

This makes branding implementation in login/register pages overridable via a SimplePageDecorator implementation.

• Sample downstream usage Add branding to login screen design-library-plugin#277

cc @dwnusbaum

Testing done

Proposed changelog entries

• Developer: make branding an extension via SimplePageDecorator.

Proposed upgrade guidelines

N/A

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[timja]

why remove these optionals?

[Vlatombe]

Since the views are moved from DefaultSimplePageDecorator to SimplePageDecorator now, they can't be missing, so just clarifying that (although it doesn't change anything functionally)

[timja]

Have you tested this with theme manager and a theme? i.e. is it going to break because simple-branding is now required?

https://github.com/jenkinsci/theme-manager-plugin/tree/c33d60e06c973ab755d815c301ad3cdd2460c258/src/main/resources/io/jenkins/plugins/thememanager/ThemeManagerSimplePageDecorator

[Vlatombe]

Works fine as the default from SimplePageDecorator will be used if none is provided by the concrete implementation.

[timja]

not tested manually but looks good, thanks!

[timja]

/label ready-for-merge

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

Thanks!