autocomplete: use select2 when selecting users to add as reviewers to PRs

This is a minimal change to PullRequestAutoComplete, inspired by Dominik Ruf.

The UX is slightly different than before, with select2 putting up an extra
input field, already before typing anything.

We use minimumInputLength 1 to get the same behaviour as before and avoid
displaying all users in a list.

The select2 widget is reused for adding more users, so when a user is selected,
we process it, close the widget, and abort the selection, waiting for the user
to enter another user name.