Fix overly aggressive prevent call

What does this MR do?

This fixes a bug where the rules that applied to the last owner of a group were too aggressive, preventing all actions.

This had non-sensical effects, such as preventing the owner of a group from reading their own group-member record, since even the policy :read_group is prevented.

This is fixed by limiting the list of prevented actions to only those we care about - specifically ones that could remove the last member and leave the group un-owned.

This ~bug was discovered during !40088 (merged), but these changes can be extracted a separate unit of work.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
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