Fix an issue with embedding slowvotes

Summary:
In some applications, using {V2} syntax to embed a vote throws. The chain of causality looks like this:

• We try to render a phabricator_form().
• This requires a CSRF token.
• We look for a CSRF token on the user.
• It's an omnipotent user with no token, so everything fails.

To resolve this, make sure we always pass the real user in.

Test Plan:

• Lots of grep.
• Made a Differential comment with {V2}.
• Made a Diffusion comment with {V2}.
• Made a Maniphest comment with {V2}.
• Replied to a Conpherence thread with {V2}.
• Created a Conpherence thread with {V2}.
• Used Conduit to update a Conpherence thread with {V2}.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley, lkassianik

Differential Revision: https://secure.phabricator.com/D8849