[DO NOT MERGE] Dough 3, all assets in /assets

Proof of concept for pulling in all front end code via bower.
- assets go in `/assets` (*not* /app/assets/my_engine)
- "loopback" symlink from `vendor/assets/loopback/my_engine` to the engine root, so the dummy app is able to reference the engine as if it has been installed via bower

Pros:
- predictable local asset paths for testing/linting
- leverages rails conventions for easy server-side handling of assets

Cons:
- more complex "where is my code coming from" in development with rubygems, bower, local path gems, and bower link all interacting in potentially "fun" ways

Usage:
In an engine - https://github.com/moneyadviceservice/pensions_calculator/pull/103
In an application - https://github.com/moneyadviceservice/frontend/pull/590

Questions:
1. if we go this route, should ALL engines be updated to work like this?
2. how about applications?