chat: handle suffixes and readers when adding username to TLF

Otherwise a valid TLF name like "a,b (conflicted copy 2019-02-14 #1)"
becomes "a,b (conflicted copy 2019-02-14 #1),a" and everything breaks.

go/chat/utils/utils.go

@@ -1646,16 +1646,32 @@ func AddUserToTLFName(g *globals.Context, tlfName string, vis keybase1.TLFVisibi
 	switch membersType {
 	case chat1.ConversationMembersType_IMPTEAMNATIVE, chat1.ConversationMembersType_IMPTEAMUPGRADE,
 		chat1.ConversationMembersType_KBFS:
+		if vis == keybase1.TLFVisibility_PUBLIC {
+			return tlfName
+		}
+
 		username := g.Env.GetUsername().String()
-		if vis != keybase1.TLFVisibility_PUBLIC {
-			if len(tlfName) == 0 {
-				tlfName = username
-			} else {
-				tlfName += "," + username
-			}
+		if len(tlfName) == 0 {
+			return username
 		}
+
+		split1 := strings.SplitN(tlfName, " ", 2) // split off suffix
+		split2 := strings.Split(split1[0], "#")   // split off readers
+		// Add the name to the writers list (assume the current user
+		// is a writer).
+		tlfName = split2[0] + "," + username
+		if len(split2) > 1 {
+			// Re-append any readers.
+			tlfName += "#" + split2[1]
+		}
+		if len(split1) > 1 {
+			// Re-append any suffix.
+			tlfName += " " + split1[1]
+		}
+		return tlfName
+	default:
+		return tlfName
 	}
-	return tlfName
 }
 
 func ForceReloadUPAKsForUIDs(ctx context.Context, g *globals.Context, uids []keybase1.UID) error {