Use organization webhook events to accept member invites about quickfeed

[JosteinLindhom]

The problem this issue references is as follows:

• Student wants to enroll in a course
• Teacher accepts student enrollment
• QuickFeed creates personal repository for student: Student-labs
• QuickFeed adds student as outside collaborator for assignment, info and Student-labs
• QuickFeed accepts outside collaborator invites on behalf of the user

At a later point, the student creates a group:

• Student creates a group studentgroup
• Teacher accepts group
• QuickFeed creates group repository studentgroup
• QuickFeed creates a GitHub team studentgroup for the group
• Student is added to the studentgroup GitHub team
  • Student is automatically invited by GitHub to the organization as a member
• QuickFeed does not accept the member invite on behalf of the user

In order for the student to see their group repository they need to be part of the studentgroup team.
For a GitHub user to be able to join a GitHub team, the user has to be a member of the organization.

To make matters worse, GitHub invites expire after 7 days. This may cause invites to expire before the student starts working on group related stuff. In these cases teachers currently have to manually re-send invites.

#1081 introduces a much simpler fix than the proposed webhook solution.
In #1081 we simply edit organization membership to member upon enrolling a student. This will invite the student to the organization as a member, and we will accept the invite on behalf of the user.

This way the student will already be a member when added to a team as part of group creation, and will be able to see their group repository without requiring further action.

[meling]

Maybe the GitHub app manifest needs to be updated to select organization.