feat(flags) add feature flag for help button

Introduces a new feature flag ('help.enabled') and uses that to
determine the visibility of the 'Help' button in a call.

react/features/base/flags/constants.js

  */
 export const FILMSTRIP_ENABLED = 'filmstrip.enabled';
 
+/**
+ * Flag indicating if the Help button should be enabled.
+ * Default: enabled (true).
+ */
+export const HELP_BUTTON_ENABLED = 'help.enabled';
+
 /**
  * Flag indicating if invite functionality should be enabled.
  * Default: enabled (true).

react/features/toolbox/components/HelpButton.js

 // @flow
 
 import { createToolbarEvent, sendAnalytics } from '../../analytics';
+import { getFeatureFlag, HELP_BUTTON_ENABLED } from '../../base/flags';
 import { translate } from '../../base/i18n';
 import { IconHelp } from '../../base/icons';
 import { connect } from '../../base/redux';
  */
 function _mapStateToProps(state: Object) {
     const { userDocumentationURL } = state['features/base/config'].deploymentUrls || {};
-    const visible = typeof userDocumentationURL === 'string';
+    const enabled = getFeatureFlag(state, HELP_BUTTON_ENABLED, true);
+    const visible = typeof userDocumentationURL === 'string' && enabled;
 
     return {
         _userDocumentationURL: userDocumentationURL,