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
Is this new functionality? If so, how is it related to import/export?