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/diff/box.tmpl

@@ -90,13 +90,13 @@
 						<div class="diff-file-box diff-box file-content {{TabSizeClass $.Editorconfig $file.Name}} gt-mt-3" id="diff-{{$file.NameHash}}" data-old-filename="{{$file.OldName}}" data-new-filename="{{$file.Name}}" {{if or ($file.ShouldBeHidden) (not $isExpandable)}}data-folded="true"{{end}}>
 							<h4 class="diff-file-header sticky-2nd-row ui top attached normal header gt-df gt-ac gt-sb">
 								<div class="diff-file-name gt-df gt-ac gt-mr-3">
-									<a role="button" class="fold-file muted gt-mr-2" {{if not $isExpandable}}style="visibility: hidden"{{end}}>
+									<button class="fold-file ui button button-ghost gt-p-0 gt-mr-3{{if not $isExpandable}} gt-invisible{{end}}">
 										{{if $file.ShouldBeHidden}}
 											{{svg "octicon-chevron-right" 18}}
 										{{else}}
 											{{svg "octicon-chevron-down" 18}}
 										{{end}}
-									</a>
+									</button>
 									<div class="gt-bold gt-df gt-ac gt-mono">
 										{{if $file.IsBin}}
 											<span class="gt-ml-1 gt-mr-3">
@@ -125,8 +125,8 @@
 										<span class="ui basic label">{{$.locale.Tr "repo.diff.protected"}}</span>
 									{{end}}
 									{{if not (or $file.IsIncomplete $file.IsBin $file.IsSubmodule)}}
-										<a class="ui basic tiny button unescape-button">{{$.locale.Tr "repo.unescape_control_characters"}}</a>
-										<a class="ui basic tiny button escape-button gt-hidden">{{$.locale.Tr "repo.escape_control_characters"}}</a>
+										<button class="ui basic tiny button unescape-button">{{$.locale.Tr "repo.unescape_control_characters"}}</button>
+										<button class="ui basic tiny button escape-button gt-hidden">{{$.locale.Tr "repo.escape_control_characters"}}</button>
 									{{end}}
 									{{if and (not $file.IsSubmodule) (not $.PageIsWiki)}}
 										{{if $file.IsDeleted}}