Implement markdown filter and editor for submission comments.

opentech/apply/activity/forms.py

...ply/activity/templates/activity/include/listing_base.html

...s/templates/funds/applicationsubmission_admin_detail.html

...y/funds/templates/funds/applicationsubmission_detail.html

opentech/static_src/src/sass/apply/components/_editor.scss

opentech/static_src/src/sass/apply/components/_feed.scss