Allow users that signed up via OAuth to set their password in order to use Git over HTTP(S)

See #1982.

![Screen Shot 2015-02-13 at 13.37.28](https://dev.gitlab.org/uploads/gitlab/gitlabhq/69fe527252/Screen_Shot_2015-02-13_at_13.37.28.png)

There's a similar tooltip for SSH: "Add an SSH key to your profile to pull or push via SSH". These are always shown on-hover, even if the persistent flash above was hidden.

cc @Sytse

See merge request !1512