Refactor Structure and Code

Created by: hixi

Considering refactor often and not doing that, we should go through another round of cleanup, roughly doing:

• remove prefix osmaxx_ from osmaxx_conversion_service package name (PRs #855 & geometalab/osmaxx-deploy#8)
• move utilities to utils (PR #861, with PRs #859 & #860 as preparation)
• rename and move conversion_api constants to conversion/constants.py (maybe a better name is needed) (PR #862)
• move rest_api to the osmaxx_conversion_service project (integrate urls.py into the project)
• osmaxx/conversion
  • reduce package nesting
• osmaxx/api_client: use django-rest-framework client instead of our own
  • remove JWT
• Harmonize imports (module level imports or class/function imports) for the individual packages (rule of thumb: Import the same package in the same style everywhere it is used)
  • PRs #863, #868, #869, ...