Add routes for user page

-   The top level routes mapping must be the last rule
-   Add fallback routes for user which login is conflict with other
    routes.
find user by login in users controller
gem 'friendly_id'
User user login in URL
cover the user page feature using feature test
Add helper method user_path to generate /:login or /u/:login

Prefer /:login, but if name is conflict, use /u/:login
merge.
add user home link in nav bar. #150