Disallow horizontal resize of form in MD theme

boards/static/css/md/base_page.css

@@ -157,6 +157,10 b' p, .br {'
     width: 100%;
 }
 
+.post-form textarea {
+    resize: vertical;
+}
+
 .form-submit {
     display: table;
     margin-bottom: 1ex;