More granular database fixtures

A lot of issues reported would benefit from a more general approach to hooking into/overriding how pytest-django's internal fixtures with regard to managing the database. This issue is a place to collect related issues that should be solved when re-thinking/re-organizing the fixtures.

The (now quite old) branch https://github.com/pytest-dev/pytest-django/tree/fixturefix is the place where I have been experimenting with ideas in this area.

Support hooking into database creation:

• #37 - alternative database setup schemes - use copy with sqlite / CREATE DATABASE foo TEMPLATE template0;
• #98 - disable xdist suffix - customize test database name
• #84 - custom sequences - make it possible to run code after test database is created

Support having database fixtures in session, module, class scope:

• #105 - Do common db setup between tests once
• #243 - Support database access for "session" level fixtures

Start by reviewing the fixturefix branch and the related issues listed in this report, especially #37, #98, #84, #105, and #243. Determine a coherent design for database-creation hooks and session, module, and class-scoped database fixtures. Done means the related use cases are supported through a reorganized fixture system.