call MW’s update.php on freshly created Wikis

this appears to be necessary for MW extensions to create their own
tables, which is not done automatically (huh?); spotted by Beuc
Merge branch 'Branch_5_1' into Branch_5_2

Conflicts:
	src/plugins/mediawiki/cronjobs/create-wikis.php
mediawiki: create-wikis.php: drop update.php debug output
Merge branch 'Branch_5_1' into Branch_5_2
use interest-noawait triggers (Debian #772870)
Merge remote-tracking branch 'origin/Branch_5_1' into Branch_5_2