Add support to mark videos as spoilers (#2255)

Add support to mark videos as spoilers (#2255)

* Add support for MSC4193: Spoilers on Media

* Clarify variable names and wording

* Restore list atom

* Improve spoilered image UX with autoload off

* Use `aria-pressed` to indicate attachment spoiler state

* Improve spoiler button tooltip wording, keep reveal button from conflicting with load errors

* Make it possible to mark videos as spoilers

* Allow videos to be marked as spoilers when uploaded

* Apply requested changes

* Show a loading spinner on spoiled media when unblurred

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Co-authored-by: Ajay Bura <32841439+ajbura@users.noreply.github.com>