Issue #1567944: Rewrite mailsystem_get_classes, etc.
Initial patch from #1508104 by znerol: Replace dynamically generated class with delegation.
Issue #1197704 by LGLC, damienvancouver: Allow multiple setting keys per mail module.
Issue #1445148 by ianchan: Fix typo that causes mailsystem_html_to_text fails on caption tag.
Issue #1423070 by Helmut Neubauer: Remove call to D7-only function drupal_theme_rebuild() from mailsystem_admin_settings() mailsystem.admin.inc
Issue #1382036 by jherencia, berdir, sgabe: Add a common way to configure the theme that will render the email.
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