MDL-54687 messages: Fix height bug with auto-rows textarea

Firefox reserves height for the horizontal scroll bars in a textarea. This
mucks up the height calculations for this element. Prevent it with overflow-x.

theme/bootstrapbase/less/moodle/forms.less

@@ -524,3 +524,7 @@ input[size] {
     padding: 0.5em;
     font-size: large;
 }
+
+textarea[data-auto-rows] {
+    overflow-x: hidden;
+}

theme/bootstrapbase/style/moodle.css

@@ -14155,6 +14155,9 @@ input[size] {
   padding: 0.5em;
   font-size: large;
 }
+textarea[data-auto-rows] {
+  overflow-x: hidden;
+}
 body.modal-open {
   overflow: hidden;
 }