Process slowvote description as remarkup (link images, activate mentions, etc)

Summary: Fixes T10361.

Test Plan:

• Created a poll with an embedded file and a mention of a task.
• Verified file was attached properly.
• Verified mention appeared on task.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10361

Differential Revision: https://secure.phabricator.com/D15277