[Request] Groups (of the non AD/LDAP variety) should be supported

[FFdhorkin]

Mattermost confusingly has two types of groups - the ones that are like Slack groups, and also AD/LDAP groups.

My workspace has a @ group created for managers. But to use the custom attributes plugin, we had to manually add the 8 people in that group, because the 'group' field is actually trying to use AD/LDAP groups, rather than Mattermost groups!

We don't want to have a multi-team workspace. So it would be great to be able to specify a group name (not a AD/LDAP group ID) and have it figure out the rest.