This exposes `Note`s on Issues & MergeRequests using a
`Types::Notes::NoteableType` in GraphQL.

Exposing notes on a new type can be done by implementing the
`NoteableType` interface on the type. The presented object should
be a `Noteable`.

Display `formatter` as the sole content of `position` object.
This means `diff_file` data is not referenced, which is the
caseu of "IOError: not opened for reading".

Having a distinct class just for that was a bit overkill

Rename classes to (hopefully) clearer names while we're doing that.

Remove superfluous lib: true, type: redis, service: true, models: true, services: true, no_db: true, api: true
Signed-off-by: NRémy Coutable <remy@rymai.me>

The initial commit doesn't have a parent, so explicitly pass the blank
SHA and handle that when calculating the position.