devilry_student: Imported Gruntjs+bower+less configs from subjectadmin.
devilry_student: Built css.
devilry_student.views.groupinvite_create: Somewhat usable but not userfriendly.
apps.core.models.groupinvite: Added restriction to avoid double invites

for the same group. See #526.
apps.core.models.groupinvite: Validate that a user is not already in a group.
apps.core.models.groupinvite: Actually add the student to the group

when they accept the invite. Handles both merge and adding candidate.