Video card menu fixes (primarily for editors)

[lbmaian]

1. Fix simultaneous arrow key navigation on video card menu and the nested channel mentions and topic dropdowns for editors. Practically this allows just using the keyboard to add channel mentions or select topic in that menu.
2. Fix video card menu keyboard navigation breaking when menu items or another video card are hovered, page is scrolled, page loses focus, and possibly other cases.
3. Fix for the first opening of the video card menu sometimes being mispositioned for editors (subsequent openings try to put menu to left of button yet still within viewport). This still doesn't fully fix the menu for videos that already have enough channel mentions (which are asynchronously loaded in) to affect the menu size, but it's good enough.
4. Change height of channel mention and topic buttons to be the same as the channel icons themselves, such that videos with 0 mentions have the same height as those with at most a single row's worth of mentions, and channel mention and topic inputs remain horizontally aligned. This helps with (3). edit: removed since conflicts with another PR that does it better
5. Fix error on channels page that resulted in video cards briefly having wrong dimensions during loading

There are some other changes that don't really change functionality - they're artifacts of the git precommit linter or leftover from previous fix attempts that made sense to keep (like moving tags/attrs around to avoid nested <a> elements).

[lbmaian]

Note on merging with #839: That PR also touches WatchQuickEditor, in particular adding @keydown.enter to the topic autocomplete. The @keydown.stop added to both the channel mentions and topic autocompletes in this PR's first commit are still required, so the topic autocomplete should have both @keydown.enter and @keydown.stop (and not naively merged into @keydown.enter.stop).