fix bug 889631 - Add signup page for external documentation sources

This adds a "/en-US/docs/mvp-signup" endpoint for submitting links.  You'll need to add a MVP_SIGNUP_EMAIL setting to your settings_local file with an email address value.

Quick note:  I haven't been able to get emails working since Luke's original email-based PRs, so please let me know if/why emails aren't being sent properly.