Restructuring

[mhauru]

• Move tests outside of dtbase
• Rename webapp to frontend
• Simplify the folder structure of backend API routers
• Move a lot of stuff from core to backend/database
• Move more stuff from core to backend, e.g. utils only used in the backend. core is now very barebones.
• Remove unused code found in the process.

Documentation has not been updated to match, because the documentation overhaul is imminent. More restructuring might be coming, but figured this could get merged already.

Closes #98 and #205, partially addresses #129

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 97.02381% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 90.65%. Comparing base (1706c7b) to head (a832c47).

:exclamation: Current head a832c47 differs from pull request most recent head 792de49. Consider uploading reports for the commit 792de49 to get more accurate results

Files	Patch %	Lines
dtbase/backend/database/utils.py	87.80%	5 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #222 +/- ## ========================================== + Coverage 89.39% 90.65% +1.25% ========================================== Files 79 79 Lines 5820 5702 -118 ========================================== - Hits 5203 5169 -34 + Misses 617 533 -84 ```

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