wire up 'sign on' buttons for questions

Clicking 'sign on' button triggers javascript AJAX call to create new signature with current_user and selected question. Upon success, javascript toggles button text, disables button, and updates signature count.

We went with the AJAX approach to benefit the user interaction on question list view. This allows users to quickly scan a question, sign on to it, and carry on. Reused this code (https://github.com/datamade/opengovernment-app/blob/master/app/assets/javascripts/sign_on_user.js) for the question show view as well.

Additionally, the signature progress bar now renders with the appropriate width % attribute.

Todo:
* need button styles for 'signed on' button
* spec: @walter what signature functionality would you like to see tested?

Closes #38