Issues Update

Builds on #120.

* Adds an Issue model, which may be related to multiple campaigns. Issues are related to projects *through* those campaigns (in our case, sector funds).
* Adds fixtures for known issue types and their associated sector funds. This also requires adding sector mappings, as they'd normally be created during the synchronization process
* Removes project listing from the landing page template
* Makes use of the vignettes on the landing template
* Includes the issue name and icon for the featured projects on the landing page