Add avatar support for GL_USER backed groups/rooms

Add avatar support for GL_USER backed groups/rooms

Part of https://gitlab.com/gitlab-org/gitter/webapp/-/issues/2397

---

• Other MR where we added avatars for GL_PROJECT: https://gitlab.com/gitlab-org/gitter/webapp/-/merge_requests/1801
• Other MR where we added avatars for GL_GROUP: https://gitlab.com/gitlab-org/gitter/webapp/-/merge_requests/1692

Testing strategy

1. Create a GL_USER community
2. Then create a room under that community

$ mongo gitter

> db.groups.insert({
        "lcUri" : "madlittlemods_gitlab",
        "sd" : {
                "extraAdmins" : [ ],
                "extraMembers" : [ ],
                "members" : "PUBLIC",
                "public" : true,
                "admins" : "GL_USER_SAME",
                "type" : "GL_USER",
                "linkPath" : "MadLittleMods",
                "externalId" : "892863"
        },
        "lcHomeUri" : "madlittlemods_gitlab/home",
        "homeUri" : "MadLittleMods_gitlab/home",
        "uri" : "MadLittleMods_gitlab",
        "name" : "MadLittleMods_gitlab"
});