Add possibilities to send a pull request to all 'sibling' repositories (BB-1631)

When I made an interesting change to a repository, I'd like all the people who also have a fork of this repository to be able to review/accept it at once.

Thus in the interface, clicking pull-request on my repository shouldn't propose me to pull from my own repository, which is quite silly, but give me the choice of all the sibling repositories and to whom I want to send my interesting changes.