Add TeamManageView and TeamInviteView CBVs

[jacobwegner]

Rewrote the invite and management views as class-based views to allow further customization by the site developer.

A couple of things to highlight:

• get_form_success_data and after_membership_added can be used to tie a newly created Membership to other business logic on a site.
• Views use the manager_required decorator. Requires remove assumptions around auth.User model #40 before this PR can land (where staff users are considered managers of all teams)

[jtauber]

On the question of "Determine if we want urls/views for SimpleTeam/SimpleMembership or just Team/Membership". I think what would typically happen in the "Simple" cases is there'd be urls/views for domain-specific objects that would involve operations on SimpleTeams/SimpleMemberships under the covers.

So for example, in Pinax LMS, adding (or inviting) a user to be an instructor on a course would, under the covers, be like adding or inviting them to a Simple Team. The CourseInstructorDetailView could subclass a SimpleTeamDetailView or could just make use of functionality provided by pinax-teams. I suggested to Greg Newman when this topic came up that we don't try to abstract prematurely and let's actually just write (for example) a one-off CourseInstructorDetailView and only factor out a base class once we have a few examples to see what the duplication of code is.

[jacobwegner]

Ok @jtauber. That sounds like a sane approach for domain-specific objects.

I'm going to update this PR to replace the FBVs for manage/invite with these CBVs and will revisit the other FBVs down the line.