Remove GWT UI

This partially reverts I9e97987428 that split reviewers plugin in two
artifacts: backend only and regular one with GWT UI. Now that the GWT
UI is removed, we can merge those artifacts together.

Also adapt the tests to changes in the plugin API.

Change-Id: Iae4531794a124180cbe203b7a9ca2b48ecbff462
34 files changed