Revert "Remove jquery_ui due to size & limited use"

This reverts commit 14e3d50c90424dbcf3b89f0d649f0657dc201d29.

Conflicts:
js/common.js

It has been established that removing jQuery-UI introduces regression
issues due to conflicts with Ace css, e.g. with plugins relying on its
functionality. This restores the library, but keeps the 'autocomplete'
switch to 'typeahead' (see 6875f99375b419559a973a5a86320597b6aec70ba).

Following the discussion in issue 0021881, an alternative approach to
cleanly remove the library has been proposed and might be merged in the
future.