Refactor/redesign Invidious frontend

[rockerBOO]

In #5130 , I spoke of some improvements to the frontend. This PR is my current attempt at this. I'm proposing these changes but probably not from this PR directly. This is due to many small issues as well as the size of this PR in general. But to open the conversation and have others try it out I am posting it here.

Key changes:

• Move towards removing pure framework (it's not completely removed) and using native CSS
• Update HTML to be more modern standard and semantic compliant
• Update font size and sizing
• Working towards theme support with some basic tests completed
• Standardizing CSS variables using CSS variables [Enhancement] CSS: Overhaul the way theming is done #5056

Current major issues:

Mobile interface is not fully working

• Controls overlay on mobile interface and not possible to change the time on the mobile interface. Maybe a matter of changing some video.js or some responsive CSS things to make the controls work better on mobile.
• Displaying many things on mobile are not laid out optimally. Removing pure framework means re-implementing these but using CSS grid to accomplish this.

Playlists management interface isn't complete

There was a lot of interfacing in the playlists management areas so getting this all working has stalled.

Some buttons are not working right

Subscribing button should show if you are subscribed/unsubscribed without refreshing the page but click on it doesn't change the visual state.

Next steps

I am currently using this every day and it feels stable enough to use as a personal version of Invidious. Issues with mobile are the biggest blocking issue so if you use mobile it might not work well for you yet.

1. Fixing up mobile
2. Playlists interface
3. Subscribe button displaying properly
4. Clean up HTML formatting with tabs/spaces in the crystal files
5. Break up the changes into smaller changes

Collaboration

Since this is a larger change it might be good to have some collaboration for testing and some development purposes.