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/css/repo.css

@@ -2943,7 +2943,7 @@ tbody.commit-list {
   padding-top: 0 !important;
 }
 
-td.blob-excerpt {
+.blob-excerpt {
   background-color: var(--color-secondary-alpha-30);
 }
 

web_src/css/review.css

@@ -21,7 +21,7 @@
   transform: scale(1.1);
 }
 
-.lines-escape a.toggle-escape-button::before {
+.lines-escape .toggle-escape-button::before {
   visibility: visible;
   content: "⚠️";
   font-family: var(--fonts-emoji);
@@ -198,7 +198,8 @@
   color: var(--color-text);
 }
 
-a.blob-excerpt {
+.code-expander-button {
+  border: none;
   color: var(--color-text-light);
   height: 28px;
   display: flex;
@@ -206,9 +207,10 @@ a.blob-excerpt {
   align-items: center;
   width: 100%;
   background: var(--color-expand-button);
+  flex: 1;
 }
 
-a.blob-excerpt:hover {
+.code-expander-button:hover {
   background: var(--color-primary);
   color: var(--color-primary-contrast);
 }