Decouple emailservice from app package (#17827)

* decouple emailservice from app package

* fix some escaped errors

* move email package under app directory

* fix i18n

* reflect review comments

app/email/errors.go

@@ -0,0 +1,13 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package email
+
+import "github.com/pkg/errors"
+
+var (
+	CreateEmailTokenError  = errors.New("could not create token")
+	NoRateLimiterError     = errors.New("the rate limit could not be found")
+	SetupRateLimiterError  = errors.New("the rate limiter could not be set")
+	RateLimitExceededError = errors.New("the rate limit is exceeded")
+)