* Make copy/paste work for source code

Fix regression casued by #12047 so copy/paste works properly in all browsers.

Fixes #12184

Also while looking at this I saw a small display issue for blame view. I think #12023 was merged into original PR through an update branch before #12047 was merged and made one of the css ruules not apply anymore.

* use pseudo-element to prevent copying of comment + symbol even when not visually selected

* remove added newline here should not be necessary anymore

* make sure empty line is newline so there is something to select and copy