Filter tasks by project, project_group, and various other fields

Tasks may now be filtered by title, priority, status, assignee,
project, project_group, story, and may be sorted and paged. Other items:

- Added tasks to mock data.
- Enabled filter mapping to understand enums and use WHERE IN() syntax.
- Fixed stories unit test.

Change-Id: I31e70ad793e5575dba2fdebbac860d3bceef0e6f