Transform to mountable engine

I have started converting the engine do be mountable, so the users can pick the console location.

The problems I am having now:

- [ ] Make a default route of `/console`, that can be easily overridden. E.g. don't require explicit `mount`.
- [x] The root route of WebEngine::Engine.routes, does not seem to be working for me ATM. Investigating...
- [x] Testing of the engine controllers...

Even though its pretty early, I'm putting this out for feedback.

/cc @guilleiguaran