Moved discussion XBlock to edx-platform
(cherry picked from commit a7c0a2a)
Backporting XBlock discussion to edx-solutions - part 1 (incomplete)
Copied test specs
Add topic selection to inline thread editing, fixing errors

Co-Authored-By: Brian Talbot <btalbot@edx.org>

TNL-549

Conflicts:
	common/static/coffee/spec/discussion/view/discussion_thread_edit_view_spec.js
	common/static/coffee/src/discussion/discussion_module_view.coffee
	common/static/coffee/src/discussion/views/discussion_thread_edit_view.js
	lms/djangoapps/django_comment_client/forum/views.py
Fixed styling issues in LMS
Enabled loading assets for Apros + various fixes
Extracted discusssion-related styling into dedicated scss file + included as an asset for discussion XBlock
Fixingi styling issues in Apros (mostly strenghtening the selector)
Fixed missing template context parameters
Fixed order of discussion js files for Apros
Fixed discussion XModule in inline mode (LMS only)