Sync removes github users that changed their username

[mark-boute]

One-sentence description

When students change their github username they are removed from the organisation by the github sync

How to reproduce the bug

1. Add github user to a project and sync to github
2. Change the user's github name on github and run the sync again
3. The user has now been removed from the github organisation

Expected behaviour

The user should be kept, as their github ID hasn't changed.