Search - making clicking the search button not submit placeholder text

Summary: Need to wire up the button to have a click handler that clears out the placeholder text. Fixes T4847.

Test Plan: Clicked the search button and got results for nothing as opposed to "Search." Typed a search and clicked button and got expected results.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4847

Differential Revision: https://secure.phabricator.com/D8960