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

go.tools.mod

@@ -7,7 +7,7 @@ require (
 	github.com/golang-migrate/migrate/v4 v4.14.1 // indirect
 	github.com/jstemmer/go-junit-report v0.9.1 // indirect
 	github.com/jteeuwen/go-bindata v3.0.7+incompatible // indirect
-	github.com/mattermost/mattermost-utilities/mmgotool v0.0.0-20210309083648-c1e5575135f9 // indirect
+	github.com/mattermost/mattermost-utilities/mmgotool v0.0.0-20210714114450-fbc82c4cf833 // indirect
 	github.com/philhofer/fwd v1.0.0 // indirect
 	github.com/reflog/struct2interface v0.6.1 // indirect
 	github.com/spf13/cobra v1.1.3 // indirect