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 We already have almost all of these styles in [discussion/views/_create-edit-post.scss](https://github.com/edx/edx-platform/blob/jsa/tnl549/lms/static/sass/discussion/views/_create-edit-post.scss#L237). Can we just make a selector in   `_create-edit-post.scss` more specific and use them instead?