Move admin to a Rails Engine

It will improve a lot the codebase. It will also allow us to have another interface design in the admin. As use Bootstrap or something else.

- [x] Create the `neighborly-admin` rails engine
- [ ] Move the model scopes that is only used on admin
- [x] Move admin controllers
- [x] Move views
- [x] Move JS
- [x] Move Styles
- [x] Move Policies
- [x] Move Locales

This PR depends of #141 and #142.