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


@talbs @polesye I'm down with leaving this per Brian's recommendation, as isolates and highlights the pain points we ultimately need to fix in the LMS.  Hopefully we will get to improving such LMS-wide issues during Q2.