Keep thumbnail guessing enabled

Thumbnail guessing causes problems on some non-WMF sites, so the
extension's default setting will be changed to disabled in
I25f52611f6f2a6db7f33d52a2fbae48c729f7686

Bug: 64554
Change-Id: Iba41a37a1d4b7957cd05516ae9466652dadcf28a
Merge "Keep thumbnail guessing enabled"