Add users filtering to the members index page

PT Story URL: https://www.pivotaltracker.com/story/show/76849484
Heroku instance: http://damp-lowlands-2351.herokuapp.com/users

Adds users filtering to the members page using JS. 
In order to follow WSO standards and enhance UX I would use something like: 
``<input autofocus="autofocus" class="form-control input-lg" type="text" id="users-filter" name="users-filter" placeholder="Filter users" >``
(BTW, there isn't a standard convention for html/css ids and classes but we have used dashes and sometimes underscores in WSO. Dashes are standard in Bootstrap.)