refactor(json-handlers): reduce code duplication

- use functions for json marshalling, request validation and errors
- this should reduce duplication and make it easier to offer multiple
  response types (e.g. html or json) in the future

data_transfer.go

@@ -30,15 +30,27 @@ func (p *passwordRegistrationRequest) Validate() error {
 	return validatePassword(p.Password)
 }
 
+func (p *passwordLoginRequest) Validate() error {
+	return nil
+}
+
 type passwordLoginRequest struct {
 	Username string `json:"username"`
 	Password string `json:"password"`
 }
 
+func (p *noneLoginRequest) Validate() error {
+	return nil
+}
+
 type noneLoginRequest struct {
 	Username string `json:"username"`
 }
 
+func (p *nfcLoginRequest) Validate() error {
+	return nil
+}
+
 type nfcLoginRequest struct {
 	Token string `json:"token"`
 }
@@ -144,6 +156,10 @@ func (r *addAuthMethodRequest) Validate() error {
 	return errors.New("invalid method")
 }
 
+func (r *changeCreditRequest) Validate() error {
+	return nil
+}
+
 type changeCreditRequest struct {
 	Diff int `json:"diff"`
 }