Add allows_individual_mappers to Group

This commit also sets up a copy of the form on the group settings page
to allow setting this attribute, and hides the button to request
individual mapper status if the group does not allow it (or the person
viewing the group detail page is already an admin)
Merge pull request #201 from azavea/feature/jcw/group-edit-mappers-text

Add allows_individual_mappers to Group