Plugin system for template widgets

Problem/Motivation

Each field on the template needs to select a widget. The widget is responsible for a few things:
- Providing an optional settings form to configure how the widget is displayed
- Rendering the widget as configured within the Template form
- Allowing a default value to configured for this template field
- ...

Currently these are just implemented as functions in a trait for each widget type, but it is time to make these into a more concrete plugin system.

Steps to reproduce

N/A

Proposed resolution

Implement plugin system

Remaining tasks

Implement

User interface changes

None

API changes

Add API for template widget plugins

Data model changes

Plugin system for widgets