Fix missing newlines in blob copy and paste in Firefox

Firefox and IE11 omit newlines when span blocks are used in the blob view. div blocks works across all browsers, so use that to fix this issue.

There are still a few surrounding newlines because of the other HTML blocks (e.g. pre and code), but this change is a vast improvement. See https://bugzilla.mozilla.org/show_bug.cgi?id=1273836 for more details.

This commit changes all references of highlighted code from span to div. There are a lot of them.

Relates to #18534 (closed)