Custom translations management: Translation file restructuring

[ValentinFutterer]

Generated from #494 after in person discussion due to the old issue getting bloated and the need to split it.

Description
We need a way for institutions (like TUG, MUG) to customize text/translations in DAMAP without editing code. Currently, translations are hardcoded in JSON files. We want admins to be able to customize translations through the admin interface, similar to how we handle custom images and themes. For this to work we first need a unified structure of our translations.

Describe the solution you'd like

• Find a suitable new naming scheme for translation keys - unique, self-explanatory
• Get feedback from data stewards
  • Show key examples and have them point out the corresponding UI element
  • After explaining key structure, show them UI elements and ask if they can predict the corresponding key names
• Combine translation files into a csv file using new naming scheme - this is later used to generate the initial database entries and also serves as documentation for modifying the frontend
  csv structure: old_key,new_key,default_value

The old german translations can be discarded.

Describe alternatives you've considered
See the old issue linked above

Additional context
Part of our ongoing work to put DAMAP in the cloud

[lpandath]

Generated view from the original idea to: #500