Pull Request: Add AutoComplete for reviewers

AutoComplete similar as code editors do it, after entering one or two letters, would ease this process a lot.

Adding reviewer(s) to a Pull Request is error prone, as the entered name(s) must match the server login name exactly, including case.

Optionally the search should match any part of the login name, not only from the beginning, as some login names follow strange rules how they are derived from the real name.