Resolve "Add the ability for users to be added on cluster level"

Summary

Changes the members input format to allow direct cluster members. If clusters are enabled by import (i.e. any line has four entries), a line with three entries will be interpreted as a direct cluster member. Direct cluster members are never added to generated clusters as we would have no way to distinguish between groups and clusters and generated cluster names are not known to the user at the moment of importing members.

Checklist

All items on this checklist should be checked.

• I have added a changelog entry.
• I have added tests for the functionality that I added.
• I adhere to the style guide.

How to review

Use a members file with the following format:

username,email,group,cluster
username,email,cluster

The second user will be added as a direct cluster member and should not be a member of any group (unless there is a separate line that adds that member to a group).

Issues and tasks closed

Closes #68