API: subscribe should return the id of the feed

[jangernert]

Creating a category will return an integer that I assume to be the id of the newly created category. Subscribing to a feed should work the same way in my opinion. That way the client can display a placeholder with the information it got locally or do a single call to /feed/get/{id}.

[Athou]

It was already the case but the documentation didn't reflect that :( Thanks!