Fix app initialization race when issuing annotation search query

[robertknight]

If the initial annotation search query was constructed
before the user's session state had been retrieved,
the groups list was empty and groups.focused() returned null
in WidgetController._loadAnnotationsFrom

This commit resolves the issue by adding a 'groups.ready()'
function which returns a promise once the groups list is ready.

It currently just returns the session.load().$promise.

Fixes #2590

[robertknight]

Talking with nick we decided on an alternative approach which is to make the main content view in the sidebar dependent on the session state being ready before it loads.