Merge branch 'feature_multiclient' into 'master'

Classes/Controller/MailController.php

Classes/Controller/QueueController.php

Classes/Domain/Model/Mail.php

Classes/Domain/Model/Template.php

Classes/Domain/Repository/MailRepository.php

Classes/Domain/Repository/TemplateRepository.php

Classes/Service/BackendService.php

Classes/Service/FormhandlerFinisherService.php

Classes/Service/MailTemplateService.php

Classes/Updates/MigrateData.php

Configuration/TCA/tx_sgmail_domain_model_mail.php

Configuration/TCA/tx_sgmail_domain_model_template.php

Configuration/TypoScript/setup.ts

Resources/Private/Language/de.locallang_db.xlf

Resources/Private/Language/locallang_db.xlf

Resources/Private/Layouts/Default.html

Resources/Private/Templates/Mail/Index.html

Resources/Private/Templates/Queue/Index.html

ext_localconf.php

ext_tables.sql