PHP7.2 replace usages of deprecated functions in core

majority of corrections were picked up by behat/unittests and, therefore, will be tested by behat/unittests (on any PHP version). This issue does not make any functional changes Couple of places that are not covered by automated tests, just do a quick walkthrough to look for regressions: Viewing diff in wiki: Create wiki module, add and edit page, view the diff of the change Workshop random allocation: Create a workshop module, add submissions as several students, as teacher move to "evaluation" phase and run "random allocation" Survey: create survey module, complete as two students, view graphs as teacher Block activity results: Create assignment module in visible group mode (with some groups defined) and grade multiple students. Add instance of block activity results pointing to this assignment, view the groups averages in this block