Replace cover URL validation with HTML attributes

[rebecca-shoptaw]

Closes #8755

Refactor. Removes unnecessary/outdated js by handling URL validation with the "url" HTML attribute.

Technical

Very straightforward! Just a quick switch-out in the add.html file and removal of unnecessary url validation js from the covers.js file.

One quick note: This issue originally arose because I noticed that the val function was causing a type error (by trying to run .trim() on an empty string), which isn't directly fixed by this adjustment. Happy to add a quick fix for that -- confirming that $(#selector).val() is not an empty string before running .trim() on it -- to this PR or a separate one, but the fix may not be necessary.

Testing

1. Go to a book page (while logged in)
2. Select Add a Cover
3. Try typing an invalid URL and hitting Submit

Screenshot

Stakeholders

@cdrini

[cdrini]

Looks good! One small fix. Putting on testing.openlibrary.org to test now...

[cdrini]

Lgtm! Tested uploading file, uploading via url, trying to upload bad url. All worked!