Show a loading animation while extension fetches static head

[rviscomi]

When the extension is slow to fetch the static head, users see a blank color bar:

There isn't any indication that work is being done, so it's unclear whether there's an error or whether to expect results soon.

Design some kind of loading animation to suggest progress being made in the background. Consider also including some kind of loading message.

One design idea is to play on the rectangle shapes of the populated color bar:

The colors can be desaturated so it's obvious that it's not the final result:

And maybe some kind of pulsing animation on random "elements"/rectangles to mesmerize the user while they wait :)

Side note: the desaturated demo image uses the pink palette in place of the default theme because it uses a linear luminance scale, which preserves the visual "sort order" of the results. Only important if the bottom row is actually sorted, otherwise we can randomize both rows.

The dynamic mode shouldn't need a loading animation because it can ~immediately process the document.head without any network latency. The snippet is also affected by the static loading delay, but we're limited in how much we can do in the console.