Make server components of inline comment content handling state-oriented

Summary: Ref T13513. Introduce a formal server-side content state object so the whole state can be saved and restored to the drafts table, read from the request, etc.

Test Plan: Created and edited inlines. Reloaded drafts with edits. Submitted normal and editing comments. Grepped for affected symbols.

Maniphest Tasks: T13513

Differential Revision: https://secure.phabricator.com/D21275