Display parsed content in the reference-editing screen for all editors

[DrewAPicture]

In #481, we added a UI for "attaching" a Trac ticket when editing parsed content is warranted, however, the contents are otherwise not displayed anywhere in the post-editing screen unless a ticket is attached.

Attached patch displays the parsed content in read-only form for anyone with the ability to view the post-editing screen. For admins, the attach controls are visible, and the action of attaching a ticket swaps out the read-only fields for editable ones.

[coffee2code]

In 942:

Code Reference: display parsed content in the reference-editing screen for all editors

fixes #679.
props DrewAPicture.