user service: initial implementation (#17668)

* conceptual user service: initial commit

* reflect review comments

* fix i18n issues and some tests

* implement get user methods

* add license

* reflect review comments

services/users/errors.go

@@ -0,0 +1,31 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package users
+
+import "errors"
+
+var (
+	AcceptedDomainError = errors.New("the email provided does not belong to an accepted domain")
+	VerifyUserError     = errors.New("could not update verify email field")
+	UserCountError      = errors.New("could not get the total number of the users.")
+)
+
+// ErrInvalidPassword indicates an error against the password settings
+type ErrInvalidPassword struct {
+	id string
+}
+
+func NewErrInvalidPassword(id string) *ErrInvalidPassword {
+	return &ErrInvalidPassword{
+		id: id,
+	}
+}
+
+func (e *ErrInvalidPassword) Error() string {
+	return "invalid password"
+}
+
+func (e *ErrInvalidPassword) Id() string {
+	return e.id
+}