Resolve "Clean up bottom status bar Web IDE"

⚠ To be merged only after !18581 (merged) is merged to not block that from being merged.

What does this MR do?

Cleans up the status bar of the WebIDE so that:

• the bar spans the entire viewport width
• the left side is reserved for the Last commit information (to be improved in following issues with pipeline).
• the right side is reserved for info regarding the currently opened file

Are there points in the code the reviewer needs to double check?

UX Support for getting the avatar of the user is coming to the API soon (https://gitlab.com/gitlab-org/gitlab-ce/issues/45821).

Until then, getting the avatar requires backend effort so it's not included in this MR. Happy to create an issue to follow up with that addition to the status bar, if you agree with this approach. ;) Issue created: https://gitlab.com/gitlab-org/gitlab-ce/issues/46111

Why was this MR needed?

Make the bar cleaner and more useful.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

Closes #44833 (closed)

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ToDo:

• Add relevant tests.