Fix an open redirect issue in Phame with "View Live"

Summary: Currently, you can set a blog URI to "evil.com" and then the live controller will issue a redirect. Instead, require a CSRF check. If it fails, pop a "this blog has moved" dialog.

Test Plan:

• Clicked "View Live" for in-app and on-domain blogs and posts.
• Hit URI directly.

{F33302}

Reviewers: vrana

Reviewed By: vrana

CC: cbg, aran

Differential Revision: https://secure.phabricator.com/D5021