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/labels/labels_selector_field.tmpl

@@ -1,10 +1,10 @@
 <div class="ui {{if or (not .HasIssuesOrPullsWritePermission) .Repository.IsArchived}}disabled{{end}} floating jump select-label dropdown">
-	<a class="text gt-df gt-ac muted">
+	<span class="text gt-df gt-ac muted">
 		<strong>{{.locale.Tr "repo.issues.new.labels"}}</strong>
 		{{if and .HasIssuesOrPullsWritePermission (not .Repository.IsArchived)}}
 			{{svg "octicon-gear" 16 "gt-ml-2"}}
 		{{end}}
-	</a>
+	</span>
 	<div class="filter menu" {{if .Issue}}data-action="update" data-issue-id="{{$.Issue.ID}}" data-update-url="{{$.RepoLink}}/issues/labels"{{else}}data-id="#label_ids"{{end}}>
 		<div class="header" style="text-transform: none;font-size:16px;">{{.locale.Tr "repo.issues.new.add_labels_title"}}</div>
 		{{if or .Labels .OrgLabels}}