Allow Branching Model prefix to use ${BITBUCKET_USER_SLUG}

We enforce that feature branches be pushed under the user's Bitbucket slug. For example, my feature branch for this story might be judkinstv/BSERV-10871. I suggest you allow the Branching model inside project and repository settings to allow the use of ${BITBUCKET_USER_SLUG} as an expandable option for feature branches.