Fix activity stream rendering bug

This commit fixes an issue in the activity stream module where
non-cached activities could be rendered before the AJAX requests for its
constituent parts had resolved. The reason this ocurred was that all
instances of the activity model referenced the same deferred in memory,
`this.prototype.dfd`, so once the first activity model resolved, all other
activities would be forced to render as well.

To reproduce bug:
* launch ngs-modules-activitystream configured with a user with at least two activities;
* make sure that those activities are not cached in localStorage; and
* verify that one of the activities is missing either an actor or an object.

To test fix:
* checkout `feature/MIE-1525` branch of modules-activitystream;
* launch ngs-modules-activitystream configured with a user with at least two activities;
* make sure that those activities are not cached in localStorage; and
* verify that the activities are all complete.

Original ticket: https://jira.nationalgeographic.com/browse/MIE-1525