Modal to "Sign-up" for un-logged in users

[swglad]

'0. On a click to "Saved" or upvote.

[swglad]

1. Modal should have the register form inside it.
2. Navbar click on "I don't have an account" should bring up the subscribe modal.
3. Set Flask-Security Register Template to index.html, just like we did with login to kill the /login page.

[alexgerstein]

Also:

4) Adding feed urls.