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/certificate/options.tpl.html

@@ -257,7 +257,7 @@
         <div class="col-sm-2">
           <div class="checkbox">
             <label>
-              <input type="checkbox" ng-model="certificate.extensions.custom.isCritical">Critical
+              <input type="checkbox" ng-model="certificate.customIsCritical">Critical
             </label>
           </div>
         </div>
@@ -270,7 +270,7 @@
             <td>{{ custom.oid }}</td>
             <td>{{ custom.encoding }}</td>
             <td>{{ custom.value }}</td>
-            <td>{{ custom.isCritical }}</td>
+            <td>{{ custom.isCritical ? "critical" : "" }}</td>
             <td>
               <button type="button" ng-click="certificate.removeCustom($index)"
                       class="btn btn-danger btn-sm pull-right">Remove