Filter stories by project_id, status, project_group_id, more.

This patch adds various new API filtering options to the browse
endpoint, so that we can build more refined searches. It also
fixes a bug where StorySummary instances without any tasks
would simply not show up via the API.

Functional tests and additional mock test data is provided.

Change-Id: Ibde6cf37dc2c4511787d94f7540d8b021a6d348b