Make symbol linking more lenient.

Summary:
Sometimes a symbol has a nested <span> in it. Clicks on that
should count as clicks on the symbol. So keep looking for symbols among
the ancestors of the click target.

This is a silly method because I don't know if there's a more idiomatic
way to do it in Javelin.

Test Plan:
Open a Differential revision where part of a symbol is
highlighted. Click on the highlighted part. Symbol search opens.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Maniphest Tasks: T1577

Differential Revision: https://secure.phabricator.com/D3113