Feature/refactor conversion

Created by: das-g

Simplifying stuff a bit by using an implicit interface for the format-specific conversion packages: They now must provide a perform_export function taking the argument they need as keyword arguments + accepting arbitrary additional keyword arguments.

Note that, if I'm not mistaken, all these changes are worker-side, thus this should be a non-breaking change, as the contract for the queue content should remain the same. As always, a deployment of all containers is recommended anyways when this change is released.

This refactoring adds a bit boilerplate code to the individual format-specific conversion modules. This was done to keep this refactoring as localized as possible and that redundancy might be removed again in further refactorings.