Add model helpers to fetch reverse dependency gems

These helpers provide a way to collect versions or rubygems which depend
on a specific rubygem. They can be used to query the database for all the
gems which contain a given dependency.
Reuse a helper method to dry the code
Add 'reverse_dependencies' controller action
Provide tests for reverse_dependencies model helpers
Add full_name field to the reverse_dependencies payload
Add reverse_dependencies action for rubygems, shorter output version