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
It is related to export. It is now possible to list the parts you want to export.