Spike: Implement "skeleton screens" to improve perceived performance

Created by: umpox

Overview of skeleton screens: https://uxdesign.cc/what-you-should-know-about-skeleton-screens-a820c45a571a

Skeleton screens are blank pages that are progressively populated with content, such as text and images, as they become available (i.e. when network latency allows). Grey or neutral-toned filled shapes, commonly called placeholders, meet the user instantly upon user interaction with calls to action or links. The placeholders (the so-called “bones” of the skeleton) are then replaced with the actual site content, and the illusion is complete. That’s what skeleton screens do: create the illusion of an instant transition.

Example from how YouTube renders:

We could potentially implement something like this for pages where we often have to wait for backend content (e.g. file blob + file tree)

There is some discussion that this doesn't always help improve perceived performance (and can even reduce it depending on the content). I think this requires further input from @sourcegraph/design