Registration, Login controllers are misleading

[askvortsov1]

These are located in the forum namespace, but return a JSON-formatted response (and JSON-formatted validation errors). However, the error handler for their pipeline uses the View/Whoops formatter, so if they run into issues, errors will be returned in a way unexpected by the frontend.

• https://github.com/flarum/core/blob/efa9d38d73d5faf18b9f47eef6603e2e1dc45527/src/Forum/Controller/LogInController.php#L90-L90
• https://github.com/flarum/core/blob/efa9d38d73d5faf18b9f47eef6603e2e1dc45527/src/Forum/Controller/RegisterController.php#L71-L71

We should refactor these so that either:

• We can override the error formatter per-route.