Add headings to signin/signup blocks on signin page.

See #1986

![Screen Shot 2015-02-13 at 15.17.34](https://dev.gitlab.org/uploads/gitlab/gitlabhq/4feb067a23/Screen_Shot_2015-02-13_at_15.17.34.png)

When signup is disabled, the heading for sign in only reads "Sign in", because there is no longer a choice of Existing/New user.

I've moved the OAuth signin options to just below the signin box because it looked odd as a separate box without a heading.

cc @sytse

See merge request !1516