Sign up wizard for staff

[tommarmstrong]

Changes:

• New signup page for academics, /clatoolkit/signup
  
• New page to create unit offerings, /clatoolkit/unitofferings/new
  
  
• Updated myunits page to show number of members, a dynamic registration link for students and a link to update the unit offering (Unit Offering UI #48). Note: the registration page must now be accessed at a URI that includes the course code due to the new URI scheme used in this PR. This makes the URIs more consistent and means that students need to be invited to the course to join. Students who aren't supposed to join and that haven't been given the link shouldn't be able to sign up
  
• Updated registration page to allow users to use existing accounts. This works by showing a "Continue as user_name" button if a user is signed in already or by showing a login button under the register button
  
• New permissions model for unit offerings Change user roles for Units #47. Instead of generic Staff / Student roles, a user can be an admin of a unit offering which enables them to be able to do what a former Staff user could do for that course. Note: users who sign up with the new signup page are currently classified as Student and reference to roles still remains in some parts of the code.
• Updated login to allow for redirection back to previous page (useful when using login_required directive)
• General bug fixes esp in line with Rework registration, myunits pages #30

[zwaters]

Nice - I want to eventually "break-up" unit offerings by teaching period too (if applicable to a unit..). See: #20 - basically, if a staff user created a unit for Semester 1 2015, they would need to create another module for Semester 1 2016 (since the data is linked to the same unit code). I still haven't thought of the best way to implement this given our current data model, but this is moving some way towards that :)

[tommarmstrong]

@zwaters That makes sense. I think that in line with issues #31 and #51, all references to units should be done via the ID of the unit instead of the course code which would then allow multiple units with the same course code to work.

Also, I think that non auto incrementing IDs could be a good thing to investigate so that the URIs for things are less predictable.

[tommarmstrong]

@zwaters, I've created a new issue (#58) for this and can start work on it on Friday when I'm back at the office