Disable jQuery Migrate by default (enable via $wgIncludejQueryMigrate)

Follows-up e5502d1f and 63c36506.

Per the plan[1], after a few weeks of migration, the jQuery Migrate plugin
will be disabled in MediaWiki core.

The added configuration variable allows sysadmins to continue to use code
that is not yet compatible with jQuery 1.9 and higher (e.g. extensions
and/or gadgets that still use legacy jQuery functionality).

[1] http://www.mail-archive.com/wikitech-l@lists.wikimedia.org/msg75735.html

Bug: 44740
Change-Id: If1c9ab722c7ce025b550b457506dcdf90d3326c3
Merge "Disable jQuery Migrate by default (enable via $wgIncludejQueryMigrate)"