Proof of Concept: Enhanced native plugin mechanism

[cc-a]

This PR accompanies #636 and demonstrates a pattern for extending the existing plugin system to prevent the need to edit core coldfront files. The core idea is that by searching for files in the plugins directory and using entry points for pip installed packages it's possible to automate the discovery and loading of settings and urlpatterns.

To try it out:

• Checkout this branch.
• Pip install the example plugin I've created at cc-a/coldfront_native_plugin (pip install git+https://github.com/cc-a/coldfront_native_plugin.git).
• Run coldfront migrate and note that the migration from the plugin is applied.
• Start the development server and open http://localhost:8000/djp-plugin/.

Some thoughts:

• Under the hood this is doing a lot of what djp makes easier to do.
• I've jumped through certain hoops to make sure the existing plugins still work so some complexity .
• Things could be made simpler again by dropping support for loading third party plugins from the plugins directory and having developers pip install -e them if necessary.

[aebruno]

I'm probably missing something but this didn't work for me. coldfront showplugins gives me an error:

$ coldfront showplugins
Unknown command: 'showplugins'
Type 'coldfront help' for usage.

[cc-a]

Hey @aebruno. Apologies, showplugins is a djp command. I must have copy pasted it over from #636. I've edited the instructions.