Add GL_GROUP support to the permission settings view

Add GL_GROUP support to the permission settings view. Now you will be able to switch away from GitLab based permissions.

Part of https://gitlab.com/gitlab-org/gitter/webapp/issues/2394

Testing strategy

1. Insert a new GitLab Gitter group into Mongo

   db.groups.insert({
           "name" : "gitlab-org",
           "uri" : "gitlab-org",
           "lcUri" : "gitlab-org",
           "lcHomeUri" : "gitlab-org/home",
           "homeUri" : "gitlab-org/home",
           "sd" : {
                   "externalId" : "9970",
                   "linkPath" : "gitlab-org",
                   "public" : true,
                   "admins" : "GL_GROUP_MAINTAINER",
                   "members" : "PUBLIC",
                   "type" : "GL_GROUP"
           },
   });

2. Insert a room in that new GitLab group

   db.troupes.insert({
           "groupId" : db.groups.findOne({ "lcUri" : "gitlab-org", })._id,
           "lcUri" : "gitlab-org/gitter-internal",
           "uri" : "gitlab-org/gitter-internal",
           "sd" : {
                   "externalId" : "1540914",
                   "linkPath" : "gitlab-org/gitter",
                   "public" : false,
                   "admins" : "GL_GROUP_MAINTAINER",
                   "members" : "GL_GROUP_MEMBER",
                   "type" : "GL_GROUP"
           }
   });

3. Visit gitlab-org/gitter-internal signed in with your GitLab account (ensure the user you sign in with is part of the gitlab-org/gitter GitLab group)
4. Room settings dropdown -> Permissions
5. Notice it says Anyone with maintainer access to the gitlab-org/gitter project on GitLab
6. Switch to another permission type and notice the warning about not being able to switch back at the bottom of the modal