Add reviewer/group to label query

Adding ability to specify a user or group by:

* Adding a ',' after the label name followed by a reviewer id
  or 'user=' and the user id.
  e.g., label:CodeReview=1,jsmith
        label.CodeReview=1,user=jsmith

* Adding a ',' after the category name followed by a group id
  or 'group=' and the group id.
  e.g., label:CodeReview=1,android_bait
        label:CodeReview=1,group=android_bait

The user specific label query is backed by the label field in Lucene.
The filtering on the group is done only after results have been
retrieved from Lucene and thus it is more expensive.

Change-Id: I39a1000712743a6ed238e70d45c0ccc1eec50031
6 files changed