Add relationships to pull requests

Currently in JIRA you can have relationships between tickets. One can block, or depend, or simply is related to another ticket. Pull requests in Stash should be capable of the same. I should be able to create a parent pull request 'group' or 'cluster' of different pull requests and define relationships between those children.

Additionally if different pull requests have the same name, aka repo_a feature/new_awesome and repo_b feature/new_awesome, they should automatically group themselves (Or offer it as an option to the reviewers/Merge manager).

Lastly, potentially if there are relationships defined between JIRA tickets (DEV-566 is blocked by DEV-555) and you are submitting pull requests for both, it seems that that relationship should follow the pull requests too. This should at least notify the merge manager of an order to merge the features (or reject them) so that your CI server build doesn't fail when Part B is merged before Part A.