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/blame.tmpl

@@ -11,8 +11,8 @@
 				{{end}}
 				<a class="ui tiny button" href="{{.RepoLink}}/src/{{.BranchNameSubURL}}/{{.TreePath | PathEscapeSegments}}">{{.locale.Tr "repo.normal_view"}}</a>
 				<a class="ui tiny button" href="{{.RepoLink}}/commits/{{.BranchNameSubURL}}/{{.TreePath | PathEscapeSegments}}">{{.locale.Tr "repo.file_history"}}</a>
-				<a class="ui tiny button unescape-button">{{.locale.Tr "repo.unescape_control_characters"}}</a>
-				<a class="ui tiny button escape-button gt-hidden">{{.locale.Tr "repo.escape_control_characters"}}</a>
+				<button class="ui tiny button unescape-button">{{.locale.Tr "repo.unescape_control_characters"}}</button>
+				<button class="ui tiny button escape-button gt-hidden">{{.locale.Tr "repo.escape_control_characters"}}</button>
 			</div>
 		</div>
 	</h4>
@@ -52,7 +52,7 @@
 							{{if $.EscapeStatus.Escaped}}
 								<td class="lines-escape">
 									{{if $row.EscapeStatus.Escaped}}
-										<a href="" class="toggle-escape-button" title="{{template "repo/diff/escape_title" dict "diff" $row "locale" $.locale}}"></a>
+										<button class="toggle-escape-button button button-ghost" title="{{template "repo/diff/escape_title" dict "diff" $row "locale" $.locale}}"></button>
 									{{end}}
 								</td>
 							{{end}}