Fire an event on received ticket branch updates to trigger reindex

It would be useful to be able to push a complete repository with a
refs/gitblit/tickets branch and have Gitblit index all those tickets.
Additionally, it would be nice for the mirroring service to recognize
an incoming update to this ref and fire an event that can be processed
by the BranchTicketService, if so configured, to reindex the tickets.
Revise push/mirror tickets branch triggering
Add and document REINDEX_TICKETS rpc request type
Improve the GO reindex-tickets shell scripts
Document ticket replication, mirroring, and advanced administration
Merged #22 "Tie mirroring, pushing, and the BranchTicketService together"
Updated the Dutch translation for 1.4.0