Merge pull request #4343 from nextcloud/enh/1467/mdn

appinfo/info.xml

appinfo/routes.php

lib/Contracts/IMailTransmission.php

lib/Controller/AccountsController.php

lib/Controller/MessagesController.php

lib/Db/Message.php

lib/Db/MessageMapper.php

lib/Migration/Version1080Date20210108093802.php

lib/Model/IMAPMessage.php

lib/Model/NewMessageData.php

lib/Service/MailManager.php

lib/Service/MailTransmission.php

src/components/Composer.vue

src/components/MdnRequest.vue

src/components/Message.vue

src/components/MessageEncryptedBody.vue

src/components/MessageHTMLBody.vue

src/components/MessagePlainTextBody.vue

src/service/MessageService.js

tests/Unit/Controller/AccountsControllerTest.php