[bugfix] Fix for '/' keyboard shortcut putting '/' into search input

Changed the search button binding from a click binding in mousetrap.js to a function binding.
Added a showSearch function that uses jquery to click the '#search-button' element
and the function returns false preventing the default action and stops the keydown event
from bubbling upwards.

Meta Discourse Bug Thread: https://meta.discourse.org/t/shortcut-for-search-leaves-a-in-the-search-field/14394
Mousetrap reference: http://craig.is/killing/mice
Merge pull request #2231 from arunisrael/fix_search_key_shortcut

[bugfix] Fix for '/' keyboard shortcut putting '/' into search input