Make embedded-relation-adapter the default

No Issue
- Generalize embedded-relation-adapter so it can be used for both fetching
  and saving records.
- Change Adapter inheritance hierarchy to
  RESTAdapter => BaseAdapter => EmbeddedRelationAdapter. ApplicationAdapter
  now extends EmbeddedRelationAdapter.
- Cleanup or remove adapters that existed just to extend directly
  from EmbeddedRelationAdapter.
Merge pull request #4721 from jaswilli/adapters

Make embedded-relation-adapter the default
Merge branch 'master' of https://github.com/TryGhost/Ghost