Prepare repo for ease-of-development

[gillesmag]

This pull request aims to improve the development environment. The following changes have been made:

• Docker
  • Add .dockerignore to avoid polluting Docker context when building images.
  • Add docker folder to consolidate Docker-related files used only for building images.
  • Mount backend code on host in backend Django container for development.
  • Make necessary changes to Dockerfiles to get development environment working after changes.
• Backend
  • Include Django database migrations in repo.
  • Move backend sources into dedicated src folder.
  • Separate Python development requirements.txt from base requirements.
  • Separate Django settings into base.py and local.py.
  • Integrate database migrations made manually with SQL (see tools/database_manager.py) into Django models/migrations.
  • Remove empty .py files.
  • Reformat backend code with gray (https://pypi.org/project/gray/).
• Frontend/client:
  • Rename client folder to frontend to clarify role of folder.
  • Use yarn instead of npm to speedup node package install.

[jdonkervliet]

Nice work, @gillesmag! Let me know if there is something you want me to look at. 😄