import/export feature

Using same JSON models as `/list` endpoint, allow import and export of backend configurations.

- Added export link and export all link. Export of specified parts can be obtained with the url `/list?partId=...`
- Added import form.

Extra
- Refactored `DBConfigsRepository` to split read and write operations
- Added styles to flash messages
Might be a good idea to add tests (or at least 1 big one) to ensure data integrity when using these `fromJsonModel` methods.