Improve accessibility when (re-)viewing files (#24817)

Visually, nothing should have changed.
Changes include
- Convert most `<a [no href]>` to `<button>` when (re-)viewing files:
- `<a [no href]>` are, by HTML definition, not a link and hence cannot
be focused
- `<a class="ui button">` can now be clicked (again?) using
<kbd>Enter</kbd>
- Previously, the installed keypress handler on `.ui.button` elements
disabled it for links somehow
- The `(un)escape file`, the `expand section` and the `expand/collapse
file` buttons can now be focused (and subsequently clicked using only
the keyboard)
- You can now press <kbd>Space</kbd> on a focused `View file` checkbox
to mark the file as viewed.
- previously, this was impossible as this checkbox listened on the wrong
event listener

The `add code comment` button has been left inaccessible for now as it
requires quite a bit of extra logic so that it is unhidden when it is
focused (you can otherwise focus it without seeing it as you are not
hovering on the corresponding line).

---------

Co-authored-by: silverwind <me@silverwind.io>

templates/repo/issue/view_title.tmpl

@@ -45,12 +45,12 @@
 			{{if .Issue.IsPull}}
 				{{$headHref := .HeadTarget|Escape}}
 				{{if .HeadBranchLink}}
-					{{$headHref = printf "<a href=\"%s\">%s</a>" (.HeadBranchLink | Escape) $headHref}}
+					{{$headHref = printf `<a href="%s">%s</a>` (.HeadBranchLink | Escape) $headHref}}
 				{{end}}
-				{{$headHref = printf "%s <a data-tooltip-content=\"%s\" data-clipboard-text=\"%s\">%s</a>" $headHref (.locale.Tr "copy_branch") (.HeadTarget | Escape) (svg "octicon-copy" 14)}}
+				{{$headHref = printf `%s <button class="button button-ghost" data-tooltip-content="%s" data-clipboard-text="%s">%s</button>` $headHref (.locale.Tr "copy_branch") (.HeadTarget | Escape) (svg "octicon-copy" 14)}}
 				{{$baseHref := .BaseTarget|Escape}}
 				{{if .BaseBranchLink}}
-					{{$baseHref = printf "<a href=\"%s\">%s</a>" (.BaseBranchLink | Escape) $baseHref}}
+					{{$baseHref = printf `<a href="%s">%s</a>` (.BaseBranchLink | Escape) $baseHref}}
 				{{end}}
 				{{if .Issue.PullRequest.HasMerged}}
 					{{$mergedStr:= TimeSinceUnix .Issue.PullRequest.MergedUnix $.locale}}