WIP: Move conf to the backend

⚠ This patch is not incremental (but tests are green )

• Review module dynamic loading
  • Rationale: Allow for alternative data collection method or Activity models to be used (e.g collect Commutes/Food using a form instead of a survey, collect source/destination of travels)
• Move configuration in the backend -- allows to configure various staff labels used through the app.
  • Rationale: specific deployment of the app might want to use different staff structure (e.g AURA deployment use different staff structure)
  • Operator of such specific deployment become a new type of users, this patch is a first step to provide the bare minimum to allow them to (easily) tailor their deployment. It provides:
    • a way to configure staff structure as well as position labels used in commute/food survey and travels
    • a way to set the translation for the specific staff structure

As a deployer/operator:

• create the l1p5-mycustomconf.py configuration file in backend/settings/l1p5 and symlink it to l1p5.py in the same directory to activate this profile
• call python manage.py init to mirror the conf in the Database (some keys are tunable by the end user - admin)

TODOS:

• add a test in the following situation: we have data but the corresponding module is deactivated (it fails)