Restructure SchemaGenerator for easier subclassing

[IvanAnishchuk]

Restructure SchemaGenerator to allow easier subclassing. In particular, to make adding new list actions (e.g. bulk update) easy.

It's absolutely crucial for using interactive documentation in conjunction with DRF-bulk — doing otherwise requires maintaining about a hundred lines of pointless copy-paste. (In particular I need it for this)

The whole changeset is basically just moving is_custom_action and is_list_view functions to the class so they can be overridden and use class attributes in conditions instead of hardcoded values. Shouldn't break anything unless those functions are used directly somewhere in the code (however I could provide compatibility just if required).

[carltongibson]

This is superseded by #5354.

Advice now:

Subclass AutoSchema to make your required customisations. (Override get_link or any of the helper methods.)

Interested in any sticking points there.