User management

Completed feature UserManagement
https://www.pivotaltracker.com/story/show/63880384

This PR has all the latest changes from previous PR (Avatar feature).  The UserManagement work is in the last 5 commits.
Also, it has some changes from Mercury feature branch.  Sorry about that.  Could not take them out easily after pairing with Bryan and Pavel.

Two scenarios for use sign_up will fails because User model has validations for two new fields: first_name and last_name

All changes deployed to yaro.herokuapp.com