Support for review notification levels

This is basically support for Rest API's 'notify' field on ReviewInputs.
as specified in [1].

UI changes
* Add 'Notification Level' dropdown on GerritServer allowing to
  select one the permitted values.  The default is 'ALL', which
  matches to the existing behavior of notifying all interested users.
* Add 'Notification Level' dropdown on GerritTrigger allowing to
  override the server default.

In case of multiple builds for a Gerrit event, the highest value is
chosen.

[1] https://gerrit-documentation.storage.googleapis.com/Documentation/2.7/rest-api-changes.html#set-review