Collection queries

We now support Collection queries!
Note, these are Wagtail collections for image and document organisation, rather than the GraphQLCollection field type.

New features

• Support Wagtail 2.11. Further testing welcome!
• Add collections types (#111). Thanks @fabienheureux
• Add hook for registering query mixins (#132). See documentation. @zerolab
• Add webpquality support for renditions (#130). Thanks @ruisaraiva19

Fixes

• Fix StubModel already registered RuntimeWarning (#133). @zerolab
• Add .live().public() to all relevant methods (#129). Thanks @mmmente
• Various tidy ups