MW 1.30.0+ is now explicitly required; as such, dropped pre-1.17 support
from AJAXPoll#onLoadExtensionSchemaUpdates.
Also "fixed" T163625 by changing an INSERT INTO query into INSERT IGNORE
INTO, swapped some "double quotes" to 'single quotes', refactored some
stuff, etc.
Bug: T163625
Change-Id: Ifbabf3bf5da25c602fe5873539520c517573890c