Fix mentions interfering with TOF render

[acholyn]

The quill mentions module seems to interfere with the rendering of the quill editor, though it isn't clear why.

Presently it's been disabled on production until we find a solution.

Potentially tangentally related - sometimes when adding text to a TOF that already has content, instead of appending to the main content, it gets appended via the ql-cursor element (eg. in Commentary of AF 53). Unsure if this is because the last elem is <em>