New icon images for Special:PageMigration

The icon images have been changed and now, all of the are of the same
gray level. Also, the icons have been reordered to have the positive
actions first. PNG fallbacks have been added as well.

Bug: 65764
Change-Id: I7319fd77626ca1a34f05d15cea2aec69344172fb
Merge "New icon images for Special:PageMigration"