CRM-21093: Call ->initialize() on the CiviCRM service now that Drupal 8 module has changed

[dsnopek]

While working on something in CRM_Util_System_Drupal8, I noticed that the ::loadBootstrap() method was getting the 'civicrm' service as a means to initialized CiviCRM, however, in civicrm/civicrm-drupal#495 we moved actual initialization to the initialize() method. The joy of circular dependencies! :-)

This PR simply adds the ->initialize() call. I searched for other instances that were missing this in civicrm-core, but couldn't find any

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• CRM-21093: Move CiviCRM initialization out of service constructor (in Drupal 8) and into method

[colemanw]

@dsnopek I've merged this into master but unless we stick it in the 4.7.28-rc branch today it won't go into the 4.7.28 release. Is it a critical bug? FYI @totten

[dsnopek]

I haven't found anything that actually breaks on it, but I might just not be testing the things that caused that line to be added to that function. All the stuff we're testing will already have initialized CiviCRM before getting to that line anyway. I just happened to noticed that it was wrong when working on #11381

[colemanw]

Ok sounds like it's good as-is.