Closes #44305

Escape miletone attribute when appending to the DOM

See merge request gitlab/gitlabhq!2359

Signed-off-by: NTakuya Noguchi <takninnovationresearch@gmail.com>

Added comment for `key` usage

Changes the IDE sidebar min width to be 340px to make it more usable
after resizing.

Closes #44836

A label name can contain a `_`, `~~`, or other Markdown-significant
characters. But label references are processed _after_ Markdown processing has
run, so we can't easily fix this on the backend.

We can make it more convenient, though, by changing the frontend to escape these
characters so they aren't processed as Markdown, when we insert them from
autocomplete.

simplified SET_FILE_ACTIVE openFiles map
use .find in router so that it returns early instead of looping all the
values

pending tabs now get added into openfiles & just filtered correctly

fixes model being cached as undefined
fixed open files not being correctly mutated

clears any active files when opening pending tab