[Media] Advanced Media Viewer should have keybinds

If it did, it could replace the current gallery entirely.

Maybe something like:

key	action
tab	cycle media within post
up/down or k/j	jump to prev/next post
left/right or h/l	prev/next media
m	maximize
f	request fullscreen
i	images only
o	open in new tab

*striked items implemented by !172 (merged)

Ctrl for dodging video player issues.

Either ctrl-arrow passes through or the arrow does and ctrl-arrow gets you the original behavior so there's no need for a gallery "mode".

If it's smart enough, it could start to try prefetching once the user starts navigating.