Move "Import existing repository by URL" option to button.

The "Import existing repository URL" (with or without form visible) looked out place with the new import buttons. I moved it to a button, and moved the form to below the buttons.

@dzaporozhets Would be nice to have in 7.9 since the moved buttons are in there as well.

Before:

![Screen_Shot_2015-03-19_at_15.08.04](https://dev.gitlab.org/gitlab/gitlabhq/uploads/6ac26467ede65a4f31d004c227fdbc3e/Screen_Shot_2015-03-19_at_15.08.04.png)

After:

![Screen_Shot_2015-03-19_at_15.42.56](https://dev.gitlab.org/gitlab/gitlabhq/uploads/a73bd7d09b7a33f58067a3bd0d7f604a/Screen_Shot_2015-03-19_at_15.42.56.png)

("publically" typo has been fixed to "publicly")

See merge request !1728