Added Search Box Preferences

Summary:
Resolves T989

• users can now disable the '/' keyboard shortcut which focuses the search box
• users can now disable the jump nav functionality of the search box

Test Plan:

• verified that the '/' keyboard shortcut works with preference enabled or unset
• verified that '/' no longer has any effect and disappears from keyboard shortcuts help overlay with preference disabled
• verified that search boxes have jump nav capabilities with jump nav functionality preference unset or enabled
• verified that search boxes do not jump with jump nav preference disabled
• verified that the jump nav still works as a jump nav with jump nav preference disabled

Reviewers: epriestley

Reviewed By: epriestley

CC: simpkins, aran, epriestley, vrana

Maniphest Tasks: T989

Differential Revision: https://secure.phabricator.com/D1902