Improve privileges panel UI for record privilege groups

[tylerjmchugh]

#8741 introduced "Record Privilege Groups" which cannot own metadata but privileges for a record can be assigned to them.

Currently it is hard to identify the "Record Privilege Groups" in the privileges panel. System groups are blue and groups that the user cannot set privileges for are yellow and the checkboxes are disabled.

This PR aims to fix this issue by changing "Record Privilege Groups" to the yellow style:

The disabled checkboxes should be enough to show the user they cannot set privileges for a group

Additionally I added a legend for the colors to help the users understand what they mean:

Checklist

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