(FR #1800) Workaround strict class inheritance check

Extension:Translate explicitly checks that this is a Page Translation
message group.  Dive into the "Uncanny valley" and make this repugnant
class look even more convincingly fake :(

TODO: use duck-typing or interfaces in Extension:Translate.  Stop dealing
with message groups as class objects, use hooks instead.  and Aargh.

See https://wikimedia.mingle.thoughtworks.com/projects/online_fundraiser/cards/1800

Change-Id: I28faf8cf7e9613bb1b14bd42e550ff1abb1ebdf5