Add loading state when changing preferences in MR

Problem

When toggling between any of the user preferences in MRs (e.g. inline and side-by-side modes, single-file mode, whitespace changes), the UI can sometimes freeze for a moment with no indication of activity.

Proposal

In !62192 (comment 613190153) we explored showing a spinner in the affected areas (e.g. changing to single-file mode would show a spinner in the diffs area only), but it didn't seem very seamless — probably because there's a big “repaint”, showing a huge white rectangle and a spinner. Instead, keeping the elements where they are seems more seamless.

Perhaps there's a middle-ground between showing progress while keeping it fluid. What if we simply made the affected elements more transparent, with a “ghost” appearance? For example:

Changing file browser	Changing diffs

This “ghost” appearance reminds me of an issue I started working long ago gitlab-design#82:

Less than 1s: no progress indicator, changes appear instantly or faux-instantly (to learn more, read this and this)