Refactor user validation

Validate each user field for creation/modification via API and web UI

validator/validator.go

@@ -0,0 +1,29 @@
+// Copyright 2021 Frédéric Guillot. All rights reserved.
+// Use of this source code is governed by the Apache 2.0
+// license that can be found in the LICENSE file.
+
+package validator // import "miniflux.app/validator"
+
+import (
+	"errors"
+
+	"miniflux.app/locale"
+)
+
+// ValidationError represents a validation error.
+type ValidationError struct {
+	TranslationKey string
+}
+
+// NewValidationError initializes a validation error.
+func NewValidationError(translationKey string) *ValidationError {
+	return &ValidationError{TranslationKey: translationKey}
+}
+
+func (v *ValidationError) String() string {
+	return locale.NewPrinter("en_US").Printf(v.TranslationKey)
+}
+
+func (v *ValidationError) Error() error {
+	return errors.New(v.String())
+}