Fix an XSS issue with certain high-priority remarkup rules embedded inside lower-priority link rules

Summary:
See https://hackerone.com/reports/758002. The link rules don't test that their parameters are flat text before using them in unsafe contexts.

Since almost all rules are lower-priority than these link rules, this behavior isn't obvious. However, two rules have broadly higher priority (monospaced text, and one variation of link rules has higher priority than the other), and the latter can be used to perform an XSS attack with input in the general form ()[ [[ ... | ... ]] ] so that the inner link rule is evaluated first, then the outer link rule uses non-flat text in an unsafe way.

Test Plan:
Tested examples in HackerOne report. A simple example of broken (but not unsafe) behavior is:

[[ `x` | `y` ]]

Differential Revision: https://secure.phabricator.com/D20937