Lint Python code throughout project

[conorsch]

Right now we have a fair number of Python linting violations as reported by flake8:

$ flake8 api blog home pledges search securethenews sites --exclude 'migrations/' --count | tail -n1
118

We should step through the Python files and clean up to enforce PEP8 standards. As a checklist for progress:

• [ ] Create Makefile target for easy linting via make flake8.
• [ ] Clean up all flake8 violations until none are reported.
• [ ] Add linting command to CI to enforce standards going forward.