Feature: Restrict choices by group.

[danmarsden]

as metioned in #206, When setting up the choices within an activity, we would like the to be able to restrict the availability of certain choices based on the groups the student is in.

For example a choice "Project 1" might only be available to students in the group "Green group".
When setting up the choices, the teacher should be able to configure a specific choice to be available to all students within the course or to students in one or more groups.

To achieve this we would allow the teacher to set group mode in the activity (like other moodle activities that support groups)

If the activity is set to "visible groups" and a specific choice is set to allow more than one group, the maximum number of students in that choice would include all users from all groups.

If the activity is set to "separate groups" and a specific choice is set to allow more than one group, the maximum number of students in that choice should be per group - eg if the maximum is 2, then it should allow 2 users from each group the choice is available to.

It may be useful to mention that issue #121 and #37 also mention groups but are slightly different to this use-case.

I'm planning to submit a PR for this soon.