PEP8 fixes (1/2).

Automatically generated with `autopep8`, command:
  `autopep8 --ignore=E128,E265,E309,E501,W601 --exclude=migrations --max-line-length=99 --recursive --in-place`
PEP8 fixes (2/2).

Generated by hand.
Add flake8 config.
Delete celery_app setup.
Lint fixes.
Run flake8 as a unittest.
plugin: Register plugins by name; share reference to registry.

`plugin_registry` allows plugins to inspect other installed
plugins in an injectable fashion.
plugin: Fix regression in 838962e; add tests.