PG: autocomplete reviewers

Use identifier in reviewers.config if available in order:
1. Username
2. Email
3. Account Id

Auto-complete implementation is largely copied from:
PolyGerrits 'gr-group-members'.

TODO:
We should let the REST API return AccountInfo instead of
identifier strings. That should enable us to easily display
reviewers in the same fashion as core PolyGerrit does. F.i.
by reusing gr-account-list.

Change-Id: Iba8d7e9136d9df89e6f79631e2e4f8862dbc7d56
3 files changed