Use pipenv instead of requirements.txt

Closes #672 (closed), #631 (closed)

Short description

• Use pipenv instead of requirements.txt
• Easier usage of virtual envs
• Easier handling of dev environments (pipenv install --dev)
• Supersedes pipconflictchecker (thus closes #631 (closed))
• Supersedes safety (integrated in pipenv check)

Todo

• Fix tests
• Delete requirements.txt
• Update docs
• Figure out how pipenv check handles python_version (https://github.com/pypa/pipenv/issues/2694)
• Fix docker build
• https://github.com/thaliawww/python-thalia/pull/1
• https://github.com/thaliawww/python-thalia/pull/2

Caveats

• This does get rid of the separate production-requirements.txt and thus may require installing more C libraries (argon2, postgresql) on development systems. On the other hand, production and development will be much closer.