#2551 REST Cleanup: Categories

Trying to break the last REST details into smaller PRs for review. 

- Moved category syncing from BlogService to CategoryService
- Implemented Category sync in REST
- Implemented Category creation in REST
- Previously, we'd force-sync categories after creation. Only XML-RPC does that now as the REST call returns the newly created category
- The blog view now calls sync blog on appear. Categories (and other metadata) weren't being synced often enough

refs #2551 
You can just use @[ categories ] (no need to change, looks good anyways!)