[PATCH 0 of 4] add support for custom pull request prefix

[Thomas De Schampheleire]

This series is a reworked implementation of a previous series, adding
support for a custom pull request prefix. With this implementation, the
actual custom prefix can be added using a 'hack', an extension mechanism
proposed by Mads Kiilerich.

The first two patches do some cleanup and are not directly related.
Patch 3 adds a pull_request_nice_id property to the pull request model,
which is the method that a 'hack' could overwrite.
Patch 4 then adds support for the 'hacks' concept.