Feature to add a new unit to facilitate manual splitting for Special:PageMigration

A new '+' icon has been added to add a new empty unit after the current one to
allow manual splitting of the translation units.

Bug: 65739
Change-Id: I2e794483665e7666117d410c33058550c7dd92cc
Merge "Feature to add a new unit to facilitate manual splitting for Special:PageMigration"