More loosely coupled plugins

[tommarmstrong]

Goal is for plugins that:

1. Work without specialised (non-generic) plugin fields in the DB
2. Don't require hardcoded URLs, and instead can register their own routes dynamically
3. Have their own setup pages allowing for more configuration than just a single text field
4. Can specify their own dependancies to be installed when the plugins get installed (may have to write an install script that checks for plugin dependancies as well as using the root requirements.txt)

This should allow for people to create plugins that can be installed without modifying the core and which have a more consistent, more loosely coupled interface with the toolkit.

I'm currently working on implementing 1, 2 & 3 (and have them working as a PoC) while I develop the WordPress plugin I'm working on. I'll detail how I propose this should work in a Pull Request soon.