Override default form/edit handlers

In order to hide/show forms elements based on template contents, monkeypatch in our own versions of the ModelFrom and EditHandler.

Create a simple stub template tag for further experimentation.
Move TextFragments to wagtail_commons

Adds template tags to bootstrap parser to support embed images and internal links. A handler for rich text embedded links ('proto-page')  is added to convert reference to internal pages, that may not have yet been created by the bootstrap process, into Wagtail's default 'page' types.

Add tags for general template use to find and output text fragments as richtext.

Fixes the omission of the flag is_staff from the bootstrap_users command. Fixes various bugs in the bootstrap_images command so that it now works as expected.
Context processor that updates bootstrapped content on the fly

Refactors bootstrap_content to make it possible for consumers to parse a single contents.yml file and apply the attributes to a page object. The result is used to override the database-sourced attributes of a page if they are also defined in the corresponding contents file.