make new post view overlay-ish

...atic/common/js/discussion/views/discussion_inline_view.js

...common/js/discussion/views/discussion_thread_list_view.js

common/static/common/js/discussion/views/new_post_view.js

...on/js/spec/discussion/view/discussion_inline_view_spec.js

.../spec/discussion/view/discussion_thread_list_view_spec.js

.../common/templates/discussion/inline-discussion.underscore

...on/static/common/templates/discussion/new-post.underscore

...ssion/static/discussion/js/views/discussion_board_view.js

lms/static/sass/discussion/_discussion.scss

lms/static/sass/discussion/elements/_navigation.scss

lms/static/sass/discussion/views/_inline.scss