[#999] Show placeholders for video/images on select pages

* on specific pages that we expect user content to have mixed pages,
  pass in a proxy_insecure_content argument to the HTML cleaner / embed
  expander

* only allow the video placeholder on http pages; on https pages, it
  just shows a blank iframe because of the mixed content issue

Fixes #999.
[#999] Force http instead of https for entry previews

This allows us to display the images/videos on mixed content instead of
using placeholders. Not ideal, but necessary right now.