Ensure compatibility with Drupal 8.6.0

[pfrenssen]

We are dealing with a number of test failures on the Drupal 8.6.x development branch. These need to be solved before Drupal 8.6.0 is released.

• Exception: Warning: call_user_func() expects parameter 1 to be a valid callback, class 'Drupal\dynamic_page_cache\EventSubscriber\DynamicPageCacheSubscriber' does not have a method 'onRouteMatch'. This seems to be related to Issue #2575841: Use proper method name in DynamicPageCacheSubscriber in which this method was removed.
• SafeMarkup::format() is scheduled for removal in Drupal 9.0.0. Use \Drupal\Component\Render\FormattableMarkup.. All instances in OG have been removed, but this is deprecation warning is still thrown in other code. Possibly
• The $memory_cache parameter was added in Drupal 8.6.x and will be required in 9.0.0. This is thrown in EntityStorageBase::__construct().
• \Drupal\Component\Utility\Unicode::strlen() is deprecated in Drupal 8.6.0 and will be removed before Drupal 9.0.0. Use mb_strlen() instead. OG is not calling this deprecated method, but it is still thrown in code we depend on.

[mariano-dagostino]

Exception: Warning: call_user_func() expects parameter 1 to be a valid callback, class 'Drupal\dynamic_page_cache\EventSubscriber\DynamicPageCacheSubscriber' does not have a method 'onRouteMatch'.

This issue is really strange. I searched for 'onRouteMatch' for the entire codebase of drupal 8.6.x, og, coder, drupal/console and I was not able to find a single entry of that event.

\Drupal\Component\Utility\Unicode::strlen() is deprecated
\SafeMarkup::format() is scheduled for removal in Drupal 9.0.0

Same applies for Unicode::strlen() and SafeMarkup::format(), the only parts of the code that have those calls are the tests that check the deprecations messages are triggered.

[pfrenssen]

This has been fixed in the meantime, we're even compatible with 8.7 now.