Changes required to switch to Dokku

[iandees]

• Updates to use Pipenv instead of requirements.txt
• Updates Dockerfile to install pipenv before the rest of the code to speed up Dockerfile creation
• Switch to use Python 3.8 instead of 2.7
• Use flask-caching instead of a custom memcached thing
• Use flask-cors instead of a custom CORS thing
• Fix encoding issues with GDAL
• Use the flask-sqlalchemy session for database URL instead