[DO NOT MERGE] PoC on Ruby DSL for configurable document types

[ryanb-gds]

This commit includes a basic DSL for configurable document types. It allows us to declare forms, block content schema, settings and metadata using Ruby rather than JSON, which opens up some interesting possibilities such as:

• easily sharing field definitions across types (just declare a method in the DSL for commonly used fields such as the body field)
• using the ActiveModel API directly from inside the DSL to declare attributes and validation - no more awkward JSON validation definitions
• defining one-off Publishing API presentation logic easily from within the DSL, without having to provide a shared method for it as a payload builder
• making the config self-validating - we can raise exceptions if it's invalid so the type won't even build. Much easier to see what the problem is than with JSON schema validation, and to perform more complex types of validation