Add watch indicators

Invidious added API for fetching watch history this year, so this MR adds basic support for that. It's not the greatest design, because they don't inject whether or not you have watched a video into any API JSON yet. Right now we just fetch every video you watched on start-up. Improvements to this can come later, like not storing the whole string as we really only need to compare hashes. It also syncs whether or not you watched a video back up to your server, which is really nice (it counts you pressing on the video as "watched" for now, which is also what Invidious does). I have added a function to unwatch a video, but it's not exposed in the UI yet.

Also adds a basic watch indicator in the relevant places. The red bar is just for looks because partial watch history isn't supported yet.