Style again the OAuth login page to fit pleroma more

There's no need to keep mastodon styling when not even our mastofe uses mastodon styling. There were also some usability problems with the old one where you'd have to scroll down on mobile to see the actual "authorize" button.

I changed colors, added animations, made it a bit more responsive, made a custom checkbox.

I also changed the title to show the instance name and not just "Pleroma"

Mobile:

Desktop