Add disabled button state to rich text editor (#9930)

* add disabled css state
* conditionally apply disabled css state
* hides disabled tooltips

res/css/views/rooms/wysiwyg_composer/components/_FormattingButtons.pcss

@@ -50,6 +50,12 @@ limitations under the License.
         }
     }
 
+    .mx_FormattingButtons_disabled {
+        .mx_FormattingButtons_Icon {
+            color: $quinary-content;
+        }
+    }
+
     .mx_FormattingButtons_Icon {
         --size: 16px;
         height: var(--size);