feat: Overhaul popup menu and provider management

[spalmurray-codecov]

This PR is primarily meant to fix a bug where github enterprise URLs would not actually work due to 'github' being the hardcoded provider on API requests. While I was in here though I took it as a chance to rewrite the popup menu to address some other issues.

Goals for the new popup:

1. Better support API tokens for cloud codecov
2. Improve error messaging
3. More clearly communicate optional settings
4. Remove unnecessary dependencies

I think this PR was successful in achieving each of these goals! Have approved and iterated on design with input from Adalene.

Am currently testing everything actually works on a real Github enterprise instance and will not merge before confirming.

This is what the final result looks like:

Here's what it currently looks like for comparison:

[nicholas-codecov]

Approved, pointed out a couple of console.log's that may have gotten left behind tho not sure if you want them in or not

[ajay-sentry]

rubber stamping ✅

[spalmurray-codecov]

Was not able to test this on GHE because our instance is years out of date and updating is non-trivial. We've decided to merge as-is as this won't break anything that currently works. Will need to follow up on the GHE story should we hear from our customers that it's not working.