Fix failure to fetch groups in VitalSource

Before loading groups the client waits for the main document frame to connect in
order to be able to fetch groups associated with that frame's URL. In
VitalSource however there is no "main frame", only the current content frame, so
`GroupsService._loadGroupsForUserAndDocument` waited indefinitely.

This failure to fetch groups did not happen on the initial launch due to a
different bug in `GroupsService._loadGroupsForUserAndDocument`. The
`this_store.route()` call returned `null` during startup and hence the code did
not wait for the main frame to connect. After a subsequent login/logout however,
the `route` call returned "sidebar" and the code did wait for the main frame.

This problem also did not happen in the LMS environment because there the client
loads a set of groups specified by our LMS app, instead of fetching groups based
on the logged-in user and current document.

Solve the first problem by using the `defaultContentFrame` method to get the
frame instead of `mainFrame`. `defaultContentFrame` returns the main frame if
there is one, or the first content frame that connected otherwise.

src/sidebar/services/groups.ts

@@ -83,7 +83,7 @@ export class GroupsService {
    * the site that the user is on.
    */
   private _mainURI(): string | null {
-    return this._store.mainFrame()?.uri ?? null;
+    return this._store.defaultContentFrame()?.uri ?? null;
   }
 
   /**

src/sidebar/services/test/groups-test.js

@@ -81,7 +81,7 @@ describe('GroupsService', () => {
         focusedGroup() {
           return this.getState().groups.focusedGroup;
         },
-        mainFrame() {
+        defaultContentFrame() {
           return this.getState().frames[0];
         },
         setDefault: sinon.stub(),