Group edit form always uses default used_roles

[jennifer-richards]

Describe the issue

When editing a group, the default set of used_roles from the GroupFeatures for the group type is always used. A group's custom used_roles should take precedence.

This is probably caused by incorrect logic here:

datatracker/ietf/group/forms.py

Line 85 in eda494a

self.used_roles = group_features.default_used_roles

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