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>

web_src/js/features/pull-view-file.js

@@ -38,7 +38,7 @@ export function initViewedCheckboxListenerFor() {
     // The checkbox consists of a div containing the real checkbox with its label and the CSRF token,
     // hence the actual checkbox first has to be found
     const checkbox = form.querySelector('input[type=checkbox]');
-    checkbox.addEventListener('change', function() {
+    checkbox.addEventListener('input', function() {
       // Mark the file as viewed visually - will especially change the background
       if (this.checked) {
         form.classList.add(viewedStyleClass);