Support `WAGTAILDOCS_SERVE_METHOD` style serve choices

[rgs258]

Feature Request

Issue Summary

It will be ideal for media to be sent through a Django view, to perform privacy checks (see Collection Privacy settings) and potentially other housekeeping tasks such as hit counting.

Steps to Reproduce

...

Technical details

• See WAGTAILDOCS_SERVE_METHOD
• See Collection Privacy settings
• Note that serving could be handled by django-sendfile
• It might be ideal to refactor this package to use AbstractDocument such that it may inherit from all the work already provided in Wagtail

[rgs258]

Note that I've largely been able to accomplish delegating storage to documents (WRDSDocument subclasses AbstractDocument) :

... but this has required a refactor of AbstractMedia (which now, below the fold, composites AbstractMediaBase and AbstractMediaFieldAdditions:

[zerolab]

@rgs258 nice idea!