This replaces API::Notes::NOTEABLE_TYPES and
API::Discussions::NOTEABLE_TYPES with class methods defined in a
separate helper modules. This allows EE to extend the list of noteable
types, without having to modify the constant in-place. We can't define
these methods directly in the API classes, as they would be used before
we're able to extend them in EE.
* adds basic discussions API for issues and snippets
* reorganizes notes specs (so same tests can be used for all noteable types - issues, MRs, snippets)