Replace !placeholder with @placeholder in User module

Thanks @borisson_ feel free to grab as many as you'd like we are just laying out the plan, you are quick on the trigger:)
Are you on IRC?

Dang cross-pollination! this failed on migrate D6. I'll see how that one fairs on it's own, this may need postponing or merging with that one.

Good catch @andypost thank you, this one is good to review without any migrate integration.

Pointing out the locations for manual testing. Also there is a double escaping bug here from Html::escape() into @ placeholder

1. +++ b/core/modules/user/src/AccountForm.php
   @@ -144,11 +144,11 @@ public function form(array $form, FormStateInterface $form_state) {
   -        $current_pass_description = $this->t('Required if you want to change the %mail or %pass below. !request_new.',
   +        $current_pass_description = $this->t('Required if you want to change the %mail or %pass below. @request_new.',
   

   This is on the user account page.

2. +++ b/core/modules/user/src/Plugin/Validation/Constraint/UserMailRequired.php
   @@ -34,7 +34,7 @@ class UserMailRequired extends Constraint implements ConstraintValidatorInterfac
   -  public $message = '!name field is required.';
   +  public $message = '@name field is required.';
   

   Validation names

3. +++ b/core/modules/user/src/Plugin/Validation/Constraint/UserMailRequired.php
   @@ -73,7 +73,7 @@ public function validate($items, Constraint $constraint) {
   -      $this->context->addViolation($this->message, ['!name' => Html::escape($account->getFieldDefinition('mail')->getLabel())]);
   +      $this->context->addViolation($this->message, ['@name' => Html::escape($account->getFieldDefinition('mail')->getLabel())]);
   

   We don't need the Html::escape() here and that will likely lead to double escaping.

4. +++ b/core/modules/user/user.api.php
   @@ -122,7 +122,7 @@ function hook_user_cancel_methods_alter(&$methods) {
   -    $name = t('User !uid', array('!uid' => $account->id()));
   +    $name = t('User @uid', array('@uid' => $account->id()));
   

   This is a number so no need to test.

5. +++ b/core/modules/user/user.module
   +++ b/core/modules/user/user.module
   @@ -54,19 +54,19 @@ function user_help($route_name, RouteMatchInterface $route_match) {
   

   These can be found on the help page and should be straight forward.

@HOG last patch had 0 bytes. Could you comment of a slight indication of what you are doing for each patch?

Not sure what you are attempting in #10 or #12

#9.3 to be clear is just removing the pre-escaping done by Html::escape()

Everything else is notes on where to find things for manual testing