Add PyPI package registry

[ezekg]

Part of #493. Implements PyPI's simple package repository. Adds release engines and release packages. And as a side-effect, adds support for non-JSON API mime types.

# Install "example" package from Keygen (other packages are still redirected to PyPI)
pip install example \
  --index-url 'https://api.keygen.sh/v1/accounts/demo/engines/pypi/simple'

Post-deploy

# Add new permissions and event types
rake db:seed

# Migrate existing role permissions
rake keygen:permissions:admins:add[package.create,package.delete,package.read,package.update,engine.read]
rake keygen:permissions:environments:add[package.create,package.delete,package.read,package.update,engine.read]
rake keygen:permissions:products:add[package.create,package.delete,package.read,package.update,engine.read]
rake keygen:permissions:licenses:add[package.read,engine.read]
rake keygen:permissions:users:add[package.read,engine.read]

Ref: PEP 503.