Make inline content "state-oriented", not "string-oriented"

Summary:
Ref T13513. Currently, all the inline code passes around strings to describe content. I plan to add background music, animation effects, etc., soon. To prepare for this change, make content a state object.

This does not change any user-visible behavior, it just prepares for content to become more complicated than a single string.

Test Plan: Created, edited, submitted, cancelled, etc., comments.

Maniphest Tasks: T13513

Differential Revision: https://secure.phabricator.com/D21273