Create DateTimeItemInterface and deprecate global constants in datetime.module

I some how missed this. It needs a bit of work to catch up to the latest changes in core.

Patch in #4 no longer applies. Re-rolled.

This looks good to go!

Patch in #10 no longer applies. Re-rolled.

1. +++ b/core/modules/datetime/datetime.module
   @@ -9,16 +9,28 @@
   + * @deprecated in Drupal 8.5.x and will be removed before Drupal 9.0.x. Use
   ...
   + * @deprecated in Drupal 8.5.x and will be removed before Drupal 9.0.x. Use
   ...
   + * @deprecated in Drupal 8.5.x and will be removed before Drupal 9.0.x. Use
   

   These need to reference the change record

2. +++ b/core/modules/datetime/src/DateTimeComputed.php
   @@ -46,9 +47,9 @@ public function getValue($langcode = NULL) {
   +    $storage_format = $datetime_type === DateTimeItem::DATETIME_TYPE_DATE ? DateTimeItemInterface::DATE_STORAGE_FORMAT : DateTimeItemInterface::DATETIME_STORAGE_FORMAT;
   

   Any reason why we're mixing the concrete Item and the ItemInterface? Should we be consistent? Moving the constant up from the Item to the Interface won't break BC

3. +++ b/core/modules/datetime/src/Plugin/Field/FieldType/DateTimeItem.php
   @@ -109,10 +109,10 @@ public static function generateSampleValue(FieldDefinitionInterface $field_defin
   +      $values['value'] = gmdate(DateTimeItemInterface::DATE_STORAGE_FORMAT, $timestamp);
   ...
   +      $values['value'] = gmdate(DateTimeItemInterface::DATETIME_STORAGE_FORMAT, $timestamp);
   

   we can use self:: here

I think #18 includes a bunch of un-intentional changes (perhaps diff happened after a coding standard issue was committed). I think this is good to go once fixed to include #17 + #18's interdiff.

Re-confirmed that all the usages of the old globals have been updated to the new class constants. This is great cleanup.

This is a great improvement.

1. I thought we already deprecated all the global constants, but https://www.drupal.org/node/2831620 says "most". Should we add the newly deprecated constants to that CR as well? (We could add an "As of Drupal 8.5.x" section or something, to distinguish it from the older deprecations.)
2. Normally we would have an @trigger_error() in the code path for the deprecated code. I couldn't think of any way to do this. We don't have a section for constants on https://www.drupal.org/core/deprecation. Our Symfony friends http://symfony.com/doc/current/contributing/code/conventions.html don't have an example for this either. So nothing actionable there.
3. My initial thought was that datetime.module should use the interface to define the deprecated constants, but I see #4 already explains why that isn't possible.

I inspected the changed usages with git diff --staged --color-words="[^[:space:][:punct:]]+|[:punct:]+" and all look correct.

I also confirmed there are no remaining usages of the constants:

[ibnsina:maintainer | Tue 06:28:17] $ grep -r "DATETIME_STORAGE_TIMEZONE" *
core/modules/datetime/datetime.module:const DATETIME_STORAGE_TIMEZONE = 'UTC';
[ibnsina:maintainer | Tue 06:31:10] $ grep -r "DATETIME_DATETIME_STORAGE_FORMAT" *
core/modules/datetime/datetime.module:const DATETIME_DATETIME_STORAGE_FORMAT = 'Y-m-d\TH:i:s';
[ibnsina:maintainer | Tue 06:31:23] $ grep -r "DATETIME_DATE_STORAGE_FORMAT " *
core/modules/datetime/datetime.module:const DATETIME_DATE_STORAGE_FORMAT = 'Y-m-d';

Committed and pushed to 8.5.x. Thanks! I published the new change record; let's also update https://www.drupal.org/node/2831620. I added a reference to this issue but did not update it with the additional rows. This can be marked back to fixed once the other CR is updated.

I've added those three constants to the CR noted in #25.

Thanks @jhedstrom.