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

api4/user_test.go

@@ -3245,7 +3245,7 @@ func TestVerifyUserEmail(t *testing.T) {
 	ruser, _ := th.Client.CreateUser(&user)
 
 	token, err := th.App.Srv().EmailService.CreateVerifyEmailToken(ruser.Id, email)
-	require.Nil(t, err, "Unable to create email verify token")
+	require.NoError(t, err, "Unable to create email verify token")
 
 	_, resp := th.Client.VerifyUserEmail(token.Token)
 	CheckNoError(t, resp)