[WIP] Clean up project structure

[mimischi]

Changelog

• Get rid of unused files and folders.
• Change deploy process on Dokku from buildpacks to Dockerfile.
• Migrate from old-fasioned requirements.txt to cutting edge Pipfile (using pipenv 🍰).

To-Do

• Make sure tests pass
• Merge Migrate to Python 3.6 #60 before merging this PR
• Update README.md with proposed new usage
• Update Wiki
• Make sure deploy to Dokku actually works

Limitations

• pyup.io currently does not support Pipfile. We would need to check package security vulnerabilities ourselves. See Support Pipfiles pyupio/pyup#197

[mimischi]

I've successfully migrated django-clock to the new proposed project structure. See related commits from mimischi/django-clock#430 and mimischi/django-clock#444.

[mimischi]

Closed in favor of #225.