Allow user to select project for events with multiple projects

VOL-103 will allow associating multiple projects with an event. Therefore, when a user arrives at the Volunteer tab of an Event, she will need a mechanism by which to choose which Volunteer Project she wishes to interact with.

Because Volunteer Projects are getting their own (separate from CiviEvent) interface in CiviVolunteer 2.0 (see VOL-96), the settings form for a Volunteer Project will need to be accessible in multiple contexts; keeping them all on Volunteer's CiviEvent tab won't do.

The Event tab's job will be reduced to: listing the associated Volunteer Projects; offering links to the various Project's settings, define, assign, etc. interfaces; and adding an existing (or new) Project to Event.