disable recording checkbox after conference has started

the recording checkbox should have been disabled after the
conference has started, like other conference controls, because
you can't change the recording setting after the conference has
started.

fixes CNVS-8288

test plan:
 - create a BBB conference, recording checkbox should be
   editable.
 - edit the BBB conference, recording checkbox should still be
   editable.
 - start the BBB conference, recording checkbox should no
   longer be editable.

Change-Id: I8ee65b45448c9a469516c6c246ea2cbd354e634a
Reviewed-on: https://gerrit.instructure.com/24635
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Zach Pendleton <zachp@instructure.com>
QA-Review: Cam Theriault <cam@instructure.com>
Product-Review: Jon Willesen <jonw@instructure.com>

app/views/jst/conferences/userSettingOptions.handlebars

@@ -2,7 +2,14 @@
   {{#if isBoolean}}
     <label class="checkbox" for="web_conference_user_settings_{{field}}">
       <input name="web_conference[user_settings][{{field}}]" type="hidden" value="0">
-      <input id="web_conference_user_settings_{{field}}" name="web_conference[user_settings][{{field}}]" type="checkbox" value="1" {{#if checked}}checked{{/if}}>
+      <input
+        id="web_conference_user_settings_{{field}}"
+        name="web_conference[user_settings][{{field}}]"
+        type="checkbox"
+        value="1"
+        {{#if ../../conference_started}}disabled="disabled"{{/if}}
+        {{#if checked}}checked{{/if}}
+        >
       {{description}}
     </label>
   {{/if}}

public/javascripts/conferences.js

@@ -92,7 +92,8 @@ define([
       }
       $('.web_conference_user_settings').html(userSettingOptionsTemplate({
         settings: selected.settings,
-        conference: conferenceData
+        conference: conferenceData,
+        conference_started: !!conferenceData['started_at']
       }));
     };