drop DB before importing

[obfusk]

Split off from #1343:

• drop DB before importing
• warn user about data being dropped
• delete old images

[obfusk]

This drops the DB. But I forgot about the card images. We should probably also delete those.
But that's more complicated since we can't use the DB transaction for that and file names may be reused.

[obfusk]

Looks like the unit test is stuck again :/

[obfusk]

I forgot about the card images.

So as a result imported cards will sometimes show old images. Seems to work well otherwise in my tests.

[obfusk]

But that's more complicated since we can't use the DB transaction for that and file names may be reused.

So we'd probably have to make a list of existing files before the import. Then subtract the files after the import and delete the rest.

[obfusk]

Will look into that. But not today.

[obfusk]

I don't think the images are ever deleted currently, even when the card is deleted.

[obfusk]

I don't think the images are ever deleted currently, even when the card is deleted.

They are. I just looked in the wrong place.

[obfusk]

I added code to remove the old images.

[obfusk]

This won't delete any existing image files unless the import succeeds. But existing image files may be overwritten by a failed import.

To avoid that we'd have to e.g. save them to a temporary directory instead and move them after the import succeeds. I can make a follow-up PR for that if you'd like, but this seems like a worthwhile improvement already and I don't want to make it unnecessarily complex.