Remove wiki transfer and legacy solution link functionality

Since VISki is no longer a thing for quite a while already, we should now remove some of the functionality relying on it. The "Finished wiki transfer" checkbox is no longer useful since there is nothing to be transferred. The "Legacy Solution" link that links to the VISki page is defunct and only confuses VIS students that expect possible additional solutions there. The other FVs also don't understand what these things are for.

We keep the legacy_answer and score_legacy fields, since the Legacy Answers and Import Wiki score depend on those.

This is the first time I'm using Django Migrations, so maybe I missed something, but locally everything seems to work without errors.