Write search bolding in a way which is certainly HTML-safe

Summary:
This algorithm is tricky, and uses `phutil_safe_html()` directly, which makes it potentially unsafe.

In particular, D8859 fixes a bug with it which caused it to produce non-utf8 output. This doesn't guarantee it's a security problem, but does make it suspicious.

I don't actually see a way to break it, but rewrite it so that it's absolutely bulletproof and does not need to call `phutil_safe_html()`.

Test Plan:
{F147487}

@rugabarbo, if you have a chance, can you check if this still works for you?

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley, rugabarbo

Differential Revision: https://secure.phabricator.com/D8862