In summary interfaces, don't render very large inline remarkup details for unit test messages

Summary: Ref T10635. An install with large blocks of remarkup (4MB) in test details is reporting slow page rendering. This is expected, but I've mostly given up on fighting this unless I absolutely have to. Degrade the interface more aggressively.

Test Plan:

• Submitted a large block of test details in remarkup format.
• Before patch: they rendered inline.
• After patch: degraded display.
• Verified small blocks are not changed.

Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T10635

Differential Revision: https://secure.phabricator.com/D20970