Replace all chevrons with a single icon

This replaces all chevrons with a single icon, affecting components like the
room list, network dropdown, and rebrand modal.

Rather than having files for each direction that can get out of sync, this
change uses one direction and rotates in CSS for the others.

Fixes https://github.com/vector-im/riot-web/issues/14663

res/css/views/dialogs/_RebrandDialog.scss

@@ -53,11 +53,12 @@ limitations under the License.
 .mx_RebrandDialog_chevron::after {
     content: '';
     display: inline-block;
-    width: 24px;
-    height: 24px;
+    width: 18px;
+    height: 18px;
     mask-position: center;
     mask-size: contain;
     mask-repeat: no-repeat;
     background-color: $muted-fg-color;
-    mask-image: url('$(res)/img/feather-customised/chevron-right.svg');
+    mask-image: url('$(res)/img/feather-customised/chevron-down.svg');
+    transform: rotate(-90deg);
 }