Add ability to deactivate a user when they leave the project.

Add a inactive column to the Account object.  Use the inactive
status to disable the user's web and ssh logins, sending
emails to the user on behalf of gerrit, adding the user as a
reviewer or to a group, and making the user appear in the
"add reviewer" and group "add member" auto completion boxes.

Bug: issue 503
Change-Id: Ib002788ebf8204dfea608d9f5ac3a5cdff20f817
20 files changed