Hide aspects of the widgets that a user doesn't have permission to use

Problem/Motivation

Per @dww in #2813405-90: Add a field to view and edit content's groups (point 8):

If the current user doesn't have permission to add content to a group, the widget should hide those aspects completely, and only show current group memberships in a read-only listing.

Elaborated at #196:

Users should only see the parts of the widget that they can use. E.g. anyone can see if a given node belongs to any of the groups they're members of, but only users with sufficient privs should see the stuff about adding a node to new groups, etc. If I'm not allowed to remove it, I don't need a huge yellow warning. I just need to see the group names without any operation buttons. Hope that clarifies the intention of #90.8

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet