Change the default for wgThumbnailMinimumBucketDistance

This has no effect while $wgThumbnailBuckets is null (the default), 
but once buckets are enabled, 0 is a poor setting for the minimum
distance. 50 is arbitrary but definitely better than leaving it at 0.

Change-Id: I845af18cfd00627b26f6325c1d1a512a492a8f7c
Merge "Change the default for wgThumbnailMinimumBucketDistance"