Add keyboard shortcuts to toggle display of search bar

[coffee2code]

With #882 having requested that the search bar input field be given focus when expanded, the efficiency in conducting a search could be increased even more by introducing keyboard shortcut(s) to toggle the display of the search bar.

Proposed:

• 's' to toggle display of the search bar (unless focus is on an editable field)
• 'esc' to hide the search bar (when focus is in the search bar)

[coffee2code]

In 1329:

developer.wordpress.org: Add keyboard shortcuts to toggle display of search bar.

• 's' key toggles display of search bar unless focus is on editable element
• 'esc' key hides search bar when focus is within search bar

Fixes #885.