Labels for pull requests

It would be really useful to us to be able to add labels to pull requests, so that we can for example label which PRs should be accepted before a particular release (e.g. 3.2.1).

They would also be useful for seeing at a glance the status of the PR (for example PassedQA).

Finally they could be used to give a clue as to where a branch should be merged into (for example hotfix - some of our devs have been known to create PRs that are directed at the default branch, rather than selecting the correct hoxfix branch!) (this has been resolved in the PR view)