If you can do this in the UI, you should be able to do it in the API. If
a discussion is not a single note discussion, or it is replyable, you
can reply to it.

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.

It also renames the API::PROJECT_ENDPOINT_REQUIREMENTS constant to
API::NAMESPACE_OR_PROJECT_REQUIREMENTS

Partially addresses #47424.

Had to make changes to spec files because
stubbing methods on frozen objects is a mess
in RSpec and leads to failures:

https://github.com/rspec/rspec-mocks/issues/1190

This whitelists all existing offenses for the various CodeReuse cops, of
which most are triggered by the CodeReuse/ActiveRecord cop.

* adds basic discussions API for issues and snippets
* reorganizes notes specs (so same tests can be used for all noteable types - issues, MRs, snippets)