autocomplete: use select2 when selecting owner of repos and PRs

This is a minimal change to SimpleUserAutoComplete, 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. This field will always have an old value and
the placeholder is thus never used, but we show it instead of the default
"Please enter 1 or more character" message.

The initSelection iteration is not pretty, but no more complex than what
happens when filtering the user list, and it has the advantage of giving a nice
name/gravatar display of the current user.