Add link GitHub account action

:fork_and_knife: Currently users are just dumped to /users/:id when they are prompted to link their GitHub account and then just have to figure out how to. This adds a new action at /profile/link-github that just provides a nice big button that starts the OAuth process. Users are now dumped to this action if they attempt to sign a CLA without having a linked GitHub account.
![screen shot 2014-02-18 at 9 53 08 am](https://f.cloud.github.com/assets/316507/2196267/8c79de46-98ac-11e3-8334-9bec8d534be9.png)