Edit summary in content language at Special:PagePreparation and Special:PageMigration

The edit summary used at both the special pages is now in the
content language, i.e, the language of the wiki. Previously, it was
appearing in the user interface language in case of PagePreparation
and was hardcoded to English in case of PageMigration.

Bug: 69592
Change-Id: I91b2d4e25f7852d2bc0bfa26e934a31bf20a42ba
Merge "Edit summary in content language at Special:PagePreparation and Special:PageMigration"