make POST create user and PUT update user

server/types.go

@@ -253,11 +253,10 @@ type apiStatsResponse struct {
 	MessagesRate float64 `json:"messages_rate"` // Average number of messages per second
 }
 
-type apiUserAddRequest struct {
+type apiUserAddOrUpdateRequest struct {
 	Username string `json:"username"`
 	Password string `json:"password"`
 	Tier     string `json:"tier"`
-	Force    bool   `json:"force"` // Used to change passwords/override existing user
 	// Do not add 'role' here. We don't want to add admins via the API.
 }