fix extra newlines when copying from diff in Firefox (#7288)

* fix extra newlines when copying from diff

See https://bugzilla.mozilla.org/show_bug.cgi?id=1273836

Basically, the <pre><code> seems to add a forced newline that is not
possible to get rid of via CSS, so I replaced it with just a <code>.

Secondly, .lines-type-marker also forced a newline in the copied text,
but that was possible to get rid of via user-select.

Safari still has a extraneous newline in the copied text of unknown
origin, but this should not block stop this PR.

* simplify .line-type-marker

* fix selector

* remove erronous ^^^

* Fix empty split diff

* Fix arc-theme-green

* fix add comment

* ensure line-num is copied too

* Update templates/repo/diff/box.tmpl

Co-Authored-By: zeripath <art27@cantab.net>

* attempt to fix safari via removing <code>

* remove useless whitespace at the end of 'class'

* remove inter-tag whitespace for code <td>s

* more inter-tag removal

* final inter-tag removal

* attempt to fix empty line copy

* move and comment getLineContent

* fix golint

* make background grey for missing added code

public/less/_base.less

@@ -160,7 +160,8 @@ a {
 }
 
 pre,
-code {
+code,
+.mono {
     font: 12px @monospaced-fonts, monospace;
 
     &.raw {