First stage of the application-level Python 2 to Python 3 conve…

NOTE: ~This PR is currently has its base set tofeature/provision-py3. When that branch is merged into python3 the base for this PR should be set to python3~ The base for this branch is set to our long-running python3 branch.

Overview

A preliminary set of changes to the Python codebase that begin the conversion from Python 2 to Python 3. The application is not functional after these changes, but we want to avoid creating one massive PR that is difficult to review.

Connects #3284

Notes

These are the high-level tasks covered by the commits in this PR.

Run 2to3 and commit the results
Fix errors reported by flake8
Fix the first error blocking tests from running successfully
Add migrations required after reformatting model field choice lists.

See the individual commit messages for more detail.

Testing Instructions

Check out the feature/provision-py3 branch of otm-cloud
Check out the feature/2to3 branch of otm-addons
Run vagrant provision app
From your host run flake8 using the command below and verify that the only output is a FutureWarning (which will be resolved when we update to a new version of flake8).
```
vagrant ssh app -c "cd /opt/app/core/opentreemap; flake8 --exclude migrations,opentreemap/settings/local_settings.py *"
```
From your host run ./scripts/manage.sh shell_plus and verify that the application loads without error.

Checklist

[ ] fixup! commits have been squashed

OpenTreeMap / otm-core