Fix two global search issues

Summary:
Fixes T8016.

• Don't explode on bad UTF8, if we happen to get some for whatever reason.
• Don't put <strong> tags in the "title" attribute.

Test Plan: Faked bad UTF8, no fatal. Hovered titles, no <strong> tags.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T8016

Differential Revision: https://secure.phabricator.com/D12710