The extension views for listing extensions and configuring an extension

were far too basic and limiting. There was no way of getting extra

context into the template without storing it either on the form or the

extension.
This change adds an extra_context parameter to both the

extension_list and configure_extension views for providing this

information.
It also adds documentation (which was missing completely) and type

annotations to make it easier to know what's expected to be passed to

these views. The arguments are now expected to be passed as keyword

arguments as well, which is the default case when included directly in

URL patterns.

Unit tests pass.
Tested this with an extension.