UX: apply a catch-all for icons in RTL (#37568)

I think the original intent was to only target directional icons, but
things fall through the cracks (see
https://meta.discourse.org/t/reverse-reply-arrow-icon-for-rtl/385565) so
this commit aims to:

* apply a catch-all to icons
* use `transform: scaleX(-1);` which provides a more correct horizontal
flip

As a downside, the transform now gets applied to all icons, but this
should come at a negligible performance cost.

app/assets/stylesheets/common/base/rtl.scss

@@ -1,25 +1,7 @@
 // Right to left styles.
 .rtl {
-  .d-icon-align-right,
-  .d-icon-angles-right,
-  .d-icon-angle-right,
-  .d-icon-arrow-right,
-  .d-icon-caret-right,
-  .d-icon-chevron-right,
-  .d-icon-hand-point-right,
-  .d-icon-quote-right {
-    transform: rotate(180deg);
-  }
-
-  .d-icon-align-left,
-  .d-icon-angles-left,
-  .d-icon-angle-left,
-  .d-icon-arrow-left,
-  .d-icon-caret-left,
-  .d-icon-chevron-left,
-  .d-icon-hand-point-left,
-  .d-icon-quote-left {
-    transform: rotate(180deg);
+  [class*="d-icon-"] {
+    transform: scaleX(-1);
   }
 }