detect existing users who should not be redirected to the signup page; see https://github.com/GigaOM/legacy-pro/issues/4415
turns out we didn't need to check all those user caps. "subscriber" would do.
Merge pull request #30 from wluo/github-4415

detect existing users who should not be redirected to the signup page
Merge branch 'master' of github.com:GigaOM/go-subscriptions into github-4373