MD Editor: Fixed scroll on mobile widths

Added min-height to flex elements to ensure they properly flex within
the container rathen than adjust to content.

For #4466

resources/sass/_forms.scss

@@ -105,6 +105,7 @@
     max-width: 100%;
     flex-grow: 1;
     flex-basis: auto !important;
+    min-height: 0;
   }
   .editor-toolbar-label {
     float: none !important;
@@ -119,7 +120,6 @@
   #markdown-editor .markdown-editor-wrap:not(.active) {
     flex-grow: 0;
     flex: none;
-    min-height: 0;
   }
 }