Expose the GENERATE_HTTP_PASSWORD capability permssion.

Gerrit has a GENERATE_HTTP_PASSWORD capability but it is not exposed
as a permission in the Gerrit Access UI.  This change exposes the
GENERATE_HTTP_PASSWORD capability and restricts REST API action for
getting and generating the HTTP password.

Bug: Issue 2790
Change-Id: Ie7cfeac9090462758438ebe6710dffae4368ccd4
6 files changed