Add GL_GROUP support to header view who can join tooltip

Add GL_GROUP support to header view who can join tooltip.

We currently don't support GL_GROUP rooms but we may in the future. I could see for example a gitlab-org/gitter room backed by the https://gitlab.com/gitlab-org/gitter GitLab sub-group.

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

The message is cut-off and I don't see an immediate option to change the width on the tooltip so I am going to opt to leave it as good enough in this iteration.

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/security",
           "uri" : "gitlab-org/security",
           "sd" : {
                   "externalId" : "4955423",
                   "linkPath" : "gitlab-org/security",
                   "public" : false,
                   "admins" : "GL_GROUP_MAINTAINER",
                   "members" : "GL_GROUP_MEMBER",
                   "type" : "GL_GROUP"
           }
   });

3. Visit gitlab-org/security signed in with your GitLab account (ensure the user you sign in with is part of the gitlab-org GitLab group)
4. Hover of the title and notice it says All GitLab gitlab-org/security group members can join