Replying swaps each mention in the quoted copy for its text. Assigning
that text to outerHTML parsed it as html; replace the mention with a
text node instead.
(cherry picked from commit 6052b123d5)
The editor keeps an attachment's content as it finds it, so a hand-written
embed with a url but no href reached the editor with its markup unvalidated,
and a mention saved under Trix's editor carried the generic octet-stream
content type the editor doesn't permit and was dropped on save. Rebuilding
each attachment from its attachable renders the hardened preview partial
and restores the mention content type.
Jeremy Daer
Stanko K.R.