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.