Implement loading

[CodexAdrian]

Fixes MOD-337

[Geometrically]

For some reason for packs like cobblemon there's a huge delay before the loading indicator. Does this happen for you too? Might be a backend bug.

Some design notes:

• In places like the loading button on the instance, I think a white version of the Modrinth logo would look better instead of the green version.
• For the general design of the launcher "widget", here's what I think it should be modeled similarly to the Browser's download tray (firefox is a rlly good example).

Design:

• There should be a similar animation when showing/hiding to the avatar dropdown on the site
• I think it would look a lot better to ditch the modrinth logo and pecentage and replace it with a loading bar, and have the text which specifies what is loading be smaller.
• When a new loading starts, it would be nice to autofocus/show the loading menu and the user has to click away to hide it. Or there could be an animation of the downloading icon "jiggling" to indicate a downloading action has started.
• There should be margin between the loading card and the navbar and the edge of the screen. Right now it kinda looks jammed in there

[Geometrically]

Looks good, will wait for wyatt's loading bar changes

Fixed