Fix https://bugzilla.wikimedia.org/show_bug.cgi?id=70471, https://bugzilla.wikimedia.org/show_bug.cgi?id=70520 and https://bugzilla.wikimedia.org/show_bug.cgi?id=70523.

Note that the Special:MassMessage page already performs checks for invalid delivery lists, and the code can be adapted to complete two of the sub-tasks. See https://www.mediawiki.org/wiki/Extension:MassMessage for information on this MediaWiki extension. This is not a beginner task, though the actual coding shouldn't be very difficult since there is similar existing code.

Students are required to read Wikimedia's general instructions at https://www.mediawiki.org/wiki/Google_Code-in_2014#Instructions_for_GCI_students first.