refs: https://github.com/TryGhost/Team/issues/831
- This ultimately fixes the index.js file
- It also makes it super clear what methods in the themeService are used by the API, and which are part of the service loading logic
- It also moves the activate and init function into a single file in a way that highlights they are very similar
- They are also very similar to what happens in storage.setFromZip but that code is mixed up with storage code at the moment
