We trust staff + tl2 and up to perform edits in grace period.
Allow them significantly more edit room in grace period prior to storing
a revision.
editing_grace_period_max_diff_high_trust applies to users with tl2 and up.
So
tl0 / 1 : we store an extra revision if more than 100 chars change
tl2 and up : we store an extra revision if more than 400 chars change
We may tweak these numbers as we go.
5b6e49ae1d