Improve locations

[mhauru]

• Improve the backend API for locations. It now returns more helpful responses, that explicitly say e.g. if we are trying to insert a location or schema that already exists, rather than giving a general error.
• To support the above, improve error handling in locations.py
• On the front end
  • Simplify the handling of duplicates. Functionality on this remains mostly the same.
  • Make the new location page remember the schema you had selected after you click submit.

[codecov-commenter]

Codecov Report

Attention: 12 lines in your changes are missing coverage. Please review.

Comparison is base (7e2fba7) 86.69% compared to head (1dbb781) 86.91%.

:exclamation: Current head 1dbb781 differs from pull request most recent head 21ed586. Consider uploading reports for the commit 21ed586 to get more accurate results

Files	Patch %	Lines
dtbase/backend/api/location/routes.py	77.27%	5 Missing :warning:
dtbase/webapp/app/locations/routes.py	71.42%	4 Missing :warning:
dtbase/core/locations.py	70.00%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #165 +/- ## ========================================== + Coverage 86.69% 86.91% +0.21% ========================================== Files 70 71 +1 Lines 4759 4792 +33 ========================================== + Hits 4126 4165 +39 + Misses 633 627 -6 ```

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