Simplistic alignment based on h2 headers for Special:PageMigration

h2 headers are aligned in the order they appear in the source and target
units. If needed, blank units are added; if there is no room for aligning,
the preceding units are merged until there is enough room for alignment.
QUnit tests have also been added to verify the same.

Bug: 66162
Change-Id: I06fea8b76b01014a83c32b972b15375aa8f44e1a
Merge "Simplistic alignment based on h2 headers for Special:PageMigration"