External redirect path + base path

[bcomnes]

Related to: #102

ie, in general for the CMS, it would make sense that the base path is "/admin" and that all URLs are generated based on that (both redirects after a social login and URLs in emails for confirmation, remember me, etc)
if we had that, I would check the return_to URL in the request for an external provider and verify it like this:

1. Is the domain/protocol the same as the SiteURL on file?
2. Is the path a child of the base path

[brycekahle]

Duplicate of #102 and #112.