#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 
Not the same thing

```obj-c
NSMutableArray *mutable = [NSMutableArray array];
[mutable addObject:@1];
[mutable addObject:@2];
NSLog(@"array: %@\n@[array]: %@", mutable, @[mutable]);
```

```
array: (
    1,
    2
)
@[array]: (
        (
        1,
        2
    )
)
```