Ticket Reference handling #1048

+ Supports referencing:
  + Tickets from other tickets via comments
  + Tickets from commits on any branch
+ Common TicketLink class used for both commits and tickets
  + TicketLink is temporary and persisted to ticket as a Reference
+ Support deletion of ticket references
  + Rebasing patchsets/branches will generate new references
  + Deleting old patchsets/branches will remove the relevant references
+ Substantial testing of use cases
  + With and without patchsets, deleting, amending
  + BranchTicketService used during testing to allow end-to-end ref testing
+ Relocated common git helper functions to JGitUtils