Fix end-user-facing uses of inline CSS

[ClearlyClaire]

Mastodon currently relies on inline CSS rules quite a bit, which is often suboptimal code factorization, and overall prevents us from using stricter Content-Security Policies.

This PR removes most of the inline CSS attributes an end-user would run into with the following exceptions:

• poll vote percentage bars in the JS-less fallback, as there doesn't seem to be any decent way to do this
• the account migration banner thing, as that uses background images
• some styling on the two-factor auth screen as I'm not sure where to put that in

It also doesn't fix most of the issues specific to the admin interface.

EDIT: used the progress HTML tag for fallback for poll options, successfully tested with recent Chromium and Firefox versions, as well as with an antiquated Firefox version
EDIT#2: changed the “moved” notice to use image tags
EDIT#3: changed the 2FA styling as well

[ClearlyClaire]

No end-user-facing inline CSS should be left, which means that with this PR, unsafe-inline can be removed and not affect users.

However, the admin interface still makes use of inline CSS quite a bit.