Disable thumbnail URL guessing by default

Config change to keep it on on WMF sites:
Iba41a37a1d4b7957cd05516ae9466652dadcf28a

Bug: 64554
Change-Id: I25f52611f6f2a6db7f33d52a2fbae48c729f7686
(cherry picked from commit 7081184535770f5ad96d47ba2def8091dc7d8e76)