Eliminate usage of Zend_Mime from Magento 2 Open Source

Add Magento's own Mime class to replace usage of Zend_Mime Zend_Mime package can now be removed.

[x] Pull request has a meaningful description of its purpose
[x] All commits are accompanied by meaningful commit messages
[x] All new or changed code is covered with unit/integration tests (if applicable)
[x] All automated tests passed successfully (all builds on Travis CI are green)

magento-engcom / php-7.2-support