[Cases] Remove minor layout shift for comment image paste progress bar

[justinkambic]

Summary

We recently merged image paste functionality to the comment editor in Cases. One iterative fix I committed to was removing a very small layout shift that takes place when the upload progress bar is conditionally rendered.

This patch will change it so the progress bar is always rendered, and only its visibility is toggled by the loading state.

Before

After

[elasticmachine]

Pinging @elastic/obs-ux-management-team (Team:obs-ux-management)

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 963e782

Metrics [docs]

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
cases	1.3MB	1.3MB	+52.0B

cc @justinkambic

[kibanamachine]

Friendly reminder: Looks like this PR hasn’t been backported yet.
To create automatically backports add a backport:* label or prevent reminders by adding the backport:skip label.
You can also create backports manually by running node scripts/backport --pr 229808 locally
cc: @justinkambic

[kibanamachine]

Friendly reminder: Looks like this PR hasn’t been backported yet.
To create automatically backports add a backport:* label or prevent reminders by adding the backport:skip label.
You can also create backports manually by running node scripts/backport --pr 229808 locally
cc: @justinkambic