Limit remarkup URI protocol length to 32 characters to avoid expensive regex behavior

Summary:
Ref T13608. When searching for bare URIs in remarkup text, don't look for URIs with a protocol string longer than 32 characters.

This avoids a case where the regexp engine may be tricked into executing at O(N^2) or some similar complexity.

Test Plan:

• Applied remarkup to "AAAA..." (512KB).
• Before: 64 seconds to process.
• After: <10ms to process.
• Ran unit tests.

Maniphest Tasks: T13608

Differential Revision: https://secure.phabricator.com/D21562