Fix wrapping long code lines in UI (#5927)

Wrapping was initially added in https://github.com/go-gitea/gitea/pull/2789
but it is currently disabled because the rule that applies wrapping to
the wrap class did not have enough CSS specificity.

Resolved the issue by using a general selector that matches all code
boxes. The previous wrap class was removed because it had only been
inconsistenly applied to various code boxes and because I found no easy
way to add classes to code boxes rendered in Markdown.

Also removed a seemingly useless :before on code view and added padding
to restore the spacing.

templates/repo/diff/section_unified.tmpl

@@ -19,7 +19,7 @@
 				{{if and $.root.SignedUserID $line.CanComment $.root.PageIsPullFiles}}
 					<a class="ui green button add-code-comment add-code-comment-{{if $line.RightIdx}}right{{else}}left{{end}}" data-path="{{$file.Name}}" data-side="{{if $line.RightIdx}}right{{else}}left{{end}}" data-idx="{{if $line.RightIdx}}{{$line.RightIdx}}{{else}}{{$line.LeftIdx}}{{end}}">+</a>
 				{{end}}
-				<pre><code class="wrap {{if $highlightClass}}language-{{$highlightClass}}{{else}}nohighlight{{end}}">{{$section.GetComputedInlineDiffFor $line}}</code></pre>
+				<pre><code class="{{if $highlightClass}}language-{{$highlightClass}}{{else}}nohighlight{{end}}">{{$section.GetComputedInlineDiffFor $line}}</code></pre>
 			</td>
 		</tr>
 		{{if gt (len $line.Comments) 0}}