Downloads: In media module deleted files assigned to downloads

[blackcoder87]

Describe the bug
When creating a download you upload a file and then choose it or just choose it from the dialog. This uses the media module.
You can delete files from within the media module. The deleted file might be still assigned to a download.

Ideally a FKC should be used, but enforcing this now would delete all downloads where the assigned file got deleted in the media module.

The FKC would link these two columns:
https://github.com/IlchCMS/Ilch-2.0/blob/master/application/modules/downloads/config/config.php#L46
https://github.com/IlchCMS/Ilch-2.0/blob/master/application/modules/media/config/config.php#L43

Add an FKC or keep it as is?

To Reproduce
Steps to reproduce the behavior:

1. Go to the downloads module and create a new download.
2. Go to the medial module and delete the file you chose for the download.
3. Currently it shows that the download is not existing and previously we had an error here.

Expected behavior
Either this should be entirely impossible (FKC) or the current state.

Additional context
#878