WIP: Improve support for forms and other complex Python/Django constructs

[ababic]

Description

Provides a low-friction mechanism for adding real form instances into template contexts by looking for data definitions in YAML with a klass: path.to.SomePythonClass key/value pair. The code then attempts to import the relevant class, create an object of that type, and add it to the context.

A generic set of objects can also be defined using the PATTERN_LIBRARY_COMMON_OBJECTS setting (using a similar dict format), which will be added to all pattern library contexts.

In cases where the default instantiation behaviour just won't do, the PATTERN_LIBRARY_CUSTOM_FACTORIES setting can be used to specify alternative factories for objects of a specific type.

https://pyyaml.org/wiki/PyYAMLDocumentation supports object instantiation via the !!python/object tag, but that doesn't provide any way to have the current HttpRequest be passed as an argument to __init__() - which can be very useful in a Django context. This solution automatically passes the current HttpRequest using the request kwarg if the __init__() method accepts it.

Fixes #113.

Related: #106, #134

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have added an appropriate CHANGELOG entry