Swap to skeleton loaders from a single circular loader on MR Diffs (Changes) tab

What does this MR do?

For #290301 (closed)

This MR replaces all of our known loading states with skeleton loaders in the rough shape of the content that will replace them.

Issues

We only have loading states for:

• The whole app
• The diff files

Unfortunately, there's a long delay between when the app (e.g. the metadata) loads and when the file tree actually renders.
This results in the app showing both the tree loader and the files loader even though the tree should be able to render earlier than the files.

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The table below lists our possible loading combinations and associated notes.

State	Loaders Showing	Notes
App Loading	Pseudo-app layout with all three loaders: versions, tree, and files
Files loading	The files loader and tree loader show	In my testing, the tree never shows until the files load in any case, so we'll always show the tree loader next to the files loader to avoid a janky shift
Some files have loaded, but more batches are loading	The files that have loaded are showing, but the files loader also show below them to indicate that more files are being requested

Loader	Description
versions	Has a square, a few lines, and a rectangle; very horizontal, indicates a bar with text and controls
tree	Indicates a list; very vertical, multiple lines, plus a larger bar at the top to indicate a search input
files	Looks like code with multiple lines of various lengths, and some vertical lines indicating the gutter, plus a file header area like a title

Screenshots (strongly suggested)

	Small and/or Mobile	Larger / Desktop
With File Tree Open
With File Tree Closed

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team