[TASK] Upgrade wizard for form_legacy

Upgrades wizard scans tt_content for not deleted 'mailform' content
elements and offers to download and install old extjs based form
extension called 'form_legacy' from TER, it's git is
located at https://github.com/FriendsOfTYPO3/form_legacy

Change-Id: Ic183c00a7b54ada57cce280f558f1b502e318dda
Resolves: #79733
Releases: master
Reviewed-on: https://review.typo3.org/52249


Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Frank Naegler <frank.naegler@typo3.org>
Tested-by: Frank Naegler <frank.naegler@typo3.org>