[MM-13839] Check for password when updating the eMail (#10207)

* Check for password when updating the eMail

* Require password for email change

* Enhance unit testing

* Restructure error handling for update email path

* govet

app/authentication.go

@@ -57,7 +57,7 @@ func (a *App) CheckPasswordAndAllCriteria(user *model.User, password string, mfa
 }
 
 // This to be used for places we check the users password when they are already logged in
-func (a *App) doubleCheckPassword(user *model.User, password string) *model.AppError {
+func (a *App) DoubleCheckPassword(user *model.User, password string) *model.AppError {
 	if err := checkUserLoginAttempts(user, *a.Config().ServiceSettings.MaximumLoginAttempts); err != nil {
 		return err
 	}

app/user.go

@@ -854,7 +854,7 @@ func (a *App) UpdatePasswordAsUser(userId, currentPassword, newPassword string)
 		return err
 	}
 
-	if err := a.doubleCheckPassword(user, currentPassword); err != nil {
+	if err := a.DoubleCheckPassword(user, currentPassword); err != nil {
 		if err.Id == "api.user.check_user_password.invalid.app_error" {
 			err = model.NewAppError("updatePassword", "api.user.update_password.incorrect.app_error", nil, "", http.StatusBadRequest)
 		}