Increase Phriction subdocument limit from 50 to 250

Summary:
A few users have come into IRC asking about this: basically, we show "children + grandchildren" until you have more than 50, then we show only children. This creates a few issues:

  - This view is surprisingly useful, and becomes less useful after it folds.
  - The folding behavior and program motivation for folding isn't obvious.

In the long term, some finessier approach is probably better here (configuration, CMS stuff, etc), but for now just bump the limit up.

Test Plan: Viewed Phriction, saw child paegs.

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D6028