Limit cover image file type.

[rebecca-shoptaw]

Closes #8762.

Feature/refactor. Uses HTML accept attribute to specify acceptable cover image file types (jpg/png/gif) so that no other file types can be submitted.

Technical

Super simple! Ended up just adding the single attribute to the HTML in covers/add.html.

Testing

1. Go to a book page, select "Add Cover" or "Manage Covers"
2. Try to upload an incorrect file type (like a PDF) -- the file should be greyed out and not selectable.

Screenshot

Stakeholders

@cdrini

[cdrini]

Works like a charm! Tested on testing!