Add submodule subscriptions fetching by projects

While submodule subscriptions can be fetched by branch, some plugins
(e.g.: delete-project) would rather need to access all submodule
subscriptions of a project (regardless of the branch). Instead of
iterating over all branches of a project, and fetching the
subscription for each branch separately, we allow fetching of
subscriptions directly by projects.

Change-Id: I2780c2e9ff135c6afc582bc1fde7986ddd2f85ae
1 file changed