Autocomplete migrated to jquery-ui autocomplete

Autocomplete migrated to jquery-ui autocomplete

No longer bundled with the library (as jquery-ui is bundled with the
main software, including the autocomplete part).

Rewrites had to be made to migrate, as several API things had changed
when jquery-ui took over the library.

Currently (like before, right?) this only autocompletes in the end of
the current textarea. So you can't jump back in the middle of a text
and autocomplete a user or group. This is a serious deficiency, though
not a regression.