apps.core.assignment: Core fields for students to create project groups.

See #526.
apps.core.migrations: Migration for the new project group fields.

See #526.
apps.core.models.assignment: Docs.
devilry_student: Reorganized views into a directory.
apps.core.models.groupinvite: Added invite model for #526.
apps.core.models: Added GroupInvite.
devilry_student: Started working on views for #526.
Merge branch 'master' into students-can-organize-groups
apps.core.models.groupinvite: Improved.
devilry_student...urls: Mockup the urls for #526.
apps.core.groupinvite: Sanity tests.
apps.core.models.groupinvite: Validation.

See #526.
apps.core.models.groupinvite: Use assertRaisesRegexp for the tests.
devilry_developer...corebuilder: Support adding related users on period.
apps.core.models.groupinvite: Test that invited student in RelatedStudent.
utils.devilry_email: Renamed the function to ``send_message``, and

made the order of the arguments better for sending single messages. This
should make it VERY easy to update existing message sending code for #471.
apps.core.models.groupinvite: Implemented and tested notifications.

See #526.
devilry_student.views.groupinvite_create: Skeleton.
apps.core.models.assignment_group: Added management filters for students.
devilry_student: Imported Gruntjs+bower+less configs from subjectadmin.