shadow: Use only a single regular expression to generate and match repo/group slugs.

vcs: Do not trigger external hook in case of actions on shadow respositories.

vcs: Add flag to indicate if repository is a shadow repository.

vcs: Use correct repo name attribute and remove workaround.

pr: Unify clone url generation of shadow repository.

pr: Display link to shadow repository on pull request page.

models: Remove unused imports.

vcs: Fix missing import.

api: Add an entry for pr shadow repositories to api functions.

vcs: Get shadow repository file system path by using already existing functions. We want to have the code that generates the shadow repo file system path at a single place.