closes https://github.com/TryGhost/Team/issues/1172
Custom theme settings sync and cache population had been left to complete in the background as it wasn't essential for it to be complete for the front-end to start. However that was causing problems for the API where theme activation and custom theme settings list requests happen very close together, with the latter often not containing the theme settings data when it is expected to.
- changed `activationBridge.*` methods to `async` so they can `await` the completion of custom theme settings sync before activating a theme
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
