Create groups + user search feed

When merging this, please remember to merge sakaiproject/3akai-ux#2578

Create a search feed that searches for groups and users at the same time. This will be used in a lot of autosuggests, for example for sharing, adding members to groups, etc.

It would be ideal if we could just specify any combination of resourceTypes to include, as this will become a requirement in the new faceted search interface.