[PLT-1015] Make all Mattermost system emails Multi-Part, with both a HTML and Plain Text version. (#5698)

* Implementation to Make all Mattermost system emails Multi-Part, with both a HTML and Plain Text version

* update lib

* update code per review

* update to use the mattermost repo

app/admin.go

@@ -212,7 +212,6 @@ func TestEmail(userId string, cfg *model.Config) *model.AppError {
 			return model.NewLocAppError("testEmail", "api.admin.test_email.reenter_password", nil, "")
 		}
 	}
-
 	if user, err := GetUser(userId); err != nil {
 		return err
 	} else {