update for Django 2.0 + simplify

[sheppard]

Fixes #72, drops support for Python 2.7, and removes a bunch of rarely used functionality to improve maintainability:

• wq.db.patterns.annotate
• wq.db.patterns.file
• wq.db.patterns.locate
• wq.db.patterns.mark
• wq.db.patterns.relate
• wq.db.contrib.search
• social auth integration in wq.db.rest.auth

Most of these use cases can be better served with domain-specific models using the newer design patterns API. However, some may be recreated as standalone packages if needed.