Add topic selection to inline thread editing, fixing errors

this change puts the topic selection dropdown into the inline view for a thread when editing.  prior to this, the selector would be missing and any attempts to submit the form would fail.

this also solves a related issue where a javascript error prevented the rich text editor for the thread body from being properly rendered in the form.

@polesye @andy-armstrong please review
@jmclaus FYI

TNL-549


NOTE this keystroke seems to fix https://openedx.atlassian.net/browse/TNL-607