* only return head: null if source branch was deleted
* add URL into GetPullRequest
* TestPullRequest_APIFormat
* log error if it is not Err(Branch)NotExist
* prevent pull request to be merged when PR is a WIP
* add tests
* add helper to prepend WIP: in PR title
* move default wip prefixes into settings
* use configurable WIP prefixes in javascript and default to first one in templates
* add documentation
* add unit test on pull model
Signed-off-by: Julien Tant <julien@craftyx.fr>