Issue #1914138 by andrewbelcher: Fixed Unsettings ['#theme'] prevents from using panelizer.
Issue #1884290 by rlmumford: Added a pluggable system for who emails get sent to.
Issue #1914110 by andrewbelcher: Fixed SimplenewsSubscriber::$newsletter_ids not defined.
Issue #1911322 by rlmumford: Allow to use entity types other than node in the mail spool
Issue #1910918 by rlmumford: Fixed sending entity sources that are not nodes results in notices.
Issue #1890718 by rlmumford: Added a default Entity source class.
Issue #1891160 by yanniboi: Fixed simplenews does not list 'hidden' newsletters in the admin overview.
Issue #1886022 by yanniboi: Added property to make simplenews subscriptions available to SOLR index.
Issue #1880548 by pixelite: Fixed taxonomy terms in subscribe block/page not being translated.
Issue #1787574 by Staratel: Added Move 'Hyperlink conversion' radios to 'Email settings' fieldset and make it to depend with #states from format radio.
