Add PHX 1.6 sample app to test suite and refactor generator code to be easier to handle new Phoenix versions going forward.

At this point, I'm thinking it might be OK, and make sense, to just cut a v4.0 release of Torch that only supports Phoenix 1.6+ and the supported Elixir versions (1.9+).

The current v3 version of Torch is pretty stable at this point, and we can easily maintain it for the near future for security updates an bug fixes without much problem.

If we are going to drop backwards-compatibility and do a v4 release it would also afford us the opportunity to change other things as it will be a major release. Now, I'd prefer the upgrade path from Torch v3 to v4 for users already on Phx 1.6 or 1.5 to be as painless as possible (i.e. require no, or minimal code changes), so the core public API's and things should remain supported if possible.

• Add Phoenix 1.6 example app to test suite
• Drop generator code supporting Phx < 1.6 and/or Elixir < 1.9
• Support LiveView templates for Torch admin?