Automatically Assign group leaders

closes: CNVS-11834

This creates a way for an instructor to
assign a random student as the group leader.
It only applies when an instructor is having groups
created automatically at the time of defining
a group category.

This also take an opportunity to refactor out
some bloated code from the group_categories_controller
and move it into some separate objects
that can be more easily understood and rapidly
unit tested through all the necessary permutations
(allowing higher level integration tests to just
cover a case or two)

It ALSO removes group leadership knowledge into
it's own object so that the callbacks in other
objects are simple and the logic regarding
how to do group leadership management is in
one place.

TEST PLAN:

AUTO_DISTRIBUTION:
1) login as an instructor
2) go to the "people" tab and try to create a group
set.
3) click on the "Create [0] groups for me" radio
button; verify you now have controls for assigning
a group leader automatically and that the strategy
radio buttons are greyed out.
4) check the "Assign a group leader automatically"
checkbox; verify the 2 nested radio buttons for
"random" and "first" strategies become enabled
5) select a strategy and fill out the rest of the
form, then submit (make sure your background job
is running)
6) verify after groups are created that each group
has a leader, and that the leader is in fact a member
of the group.

SelfSignup:
1) login as an instructor
2) go to the "people" tab and try to create a group
set.
3) enable self-signup.
4) check the "Assign a group leader automatically"
checkbox; verify the 2 nested radio buttons for
"random" and "first" strategies become enabled
5) select a strategy and fill out the rest of the
form, then submit.
6) Login as a student for the same course and join
the group.
7) verify that the student has been made the group
leader.

Change-Id: I2cdd9f5ed2fd577469beec4ab7369c69ecf7eaa6
Reviewed-on: https://gerrit.instructure.com/35130
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Braden Anderson <banderson@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Ethan Vizitei <evizitei@instructure.com>