Fixing handling of adding custom OIDs in UI (#653)

* is_critical wasn't in the schema, so was getting dropped.
* isCritical in the Javascript wasn't getting assigned if it was unchecked. Now, it will be assumed false if missing.
* The display of critical or not in the list of added custom OIDs was unclear when it was just true/false with no heading. Now it will be displayed as critical or nothing instead.
* The namespace for the checkbox for isCritical was wrong, and didn't get processed with the oid/type/value variables.

lemur/static/app/angular/certificates/services.js

@@ -56,7 +56,7 @@ angular.module('lemur')
             this.extensions.custom.push(
               {
                 'oid': this.customOid,
-                'isCritical': this.customIsCritical,
+                'isCritical': this.customIsCritical || false,
                 'encoding': this.customEncoding,
                 'value': this.customValue
               }