XWIKI-21011: Mail resender script service API resends all mails when called from a subwiki
XWIKI-21015: Missing legacy instance APIs in distribution
* Also add missing EM categories
* Fix typo in MailResender.xml (in the "from" test)
* Make the resender scheduer job execute as XWiki.superadmin
* Male the MailResender.xml document customizable instead of demo to avoid deleting it (it's now an official feature)
* Add debug logs when resending mail
* Fix mail resender scheduler typo in description