Add 'TODO' keyword on the comment on Header.php

Adding TODO or FIXME keywords on comments which indicate work to do
or bugs to fix makes tracking to-dos easier
Add PHPDoc comments for the return type of the static methods in the Mail and Team classes
The 'updated' field on the matches table should match the time when the match was created/edited, not when it was played
Convert Match::calculateEloDiff() into a static function, since it doesn't depend on a match already entered in the database
Add PHPDoc for the Match class