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