Disable "Send Message" profile action if viewer is logged out

Summary: Fixes T9598.

Test Plan:

• Used "Send Message" as a logged-in user.
• Used "Send Message" as a logged-out user. The action was disabled and clicking it popped up a login dialog.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9598

Differential Revision: https://secure.phabricator.com/D14326