Multisite signup functions don't support SSL

[SergeyBiryukov]

confirm_blog_signup()​, confirm_another_blog_signup()​, and wpmu_signup_blog_notification()​ use hardcoded http:// links.

[jeremyfelt]

We probably need to check a little further before assuming SSL in some cases. If a new subdomain or domain is being created, it's not necessarily the case that SSL will be available for activation. For subdirectories, we should have no trouble assuming the same scheme as the network.

[thomaswm]

In [34790], a $blog_id parameter has been introduced in confirm_another_blog_signup(). If it is set, then the function uses home_url() and wp_login_url() to generate the necessary URLs.

We could do the same thing in wpmu_signup_blog_notification() and confirm_blog_signup().