Add support for manually entering 2FA details

> ![Screen_Shot_2015-05-22_at_6.39.26_PM](https://gitlab.com/gitlab-org/gitlab-ce/uploads/27b6cc0b2f88e0ccec234dd8d20db957/Screen_Shot_2015-05-22_at_6.39.26_PM.png)

Related to #1679 though I'm not sure if it entirely fixes it, since I don't have access to a Windows Phone.

See merge request !694