Remove allow_image_download parameter

If a corpus has image attachments, you can set the parameter `allow_image_download` to control whether the frontend interface will show a download button next to the image. (No effect for PDF attachments.)

This is a bit of a misnomer; if the user is viewing the image they have already downloaded it, and they can always save it to more permanent location with right-click -> "Save as" or such. (Which is not like, an exploit, just the intended functionality of the browser.) So this parameter does not "allow" anything, it just adds some extra convenience.

I was going to suggest that `allow_image_download` should be `True` by default (it's not), but then I could not think of any use case where you would set this to `False`. If you wanted to prevent users saving images, this will not do that, but even if it would: why? It's not like we're selling subscriptions. And if the images are valuable research material (otherwise we would not include them), a researcher should be able to save them for future reference.