[Flashcards] Delete Unused Media from the Output

[pishoyg]

Right now, our media class records all media files regardless of whether or not they actually ended up being used. This results in the output (HTML and Anki) containing lots of unused media files. Revisit.

[pishoyg]

This happens due to the cards that end up without a front or a back, so they get dropped from the package. Their media files would've been included anyway!

[pishoyg]

The dialected HTML pages will become less important as we make progress on #122, because it's more likely then that we will focus on the all-dialect HTML outputs only.
However, this will continue to be, technically, a bug.

[pishoyg]

Idea: Can you retrieve the media per card, rather than per field? 🤔

[pishoyg]

This is not problematic at the moment. We will reopen if that changes.

[pishoyg]

I changed my mind. This belongs in the backlog, not in README.