Add support for protocol relative Zencoder URLS

This change allows to configure Pageflow to use protocol relative URLs for video and audio files. This avoids mixed content warnings when viewing entires via HTTPS, but still does not burden the CDN with encryption if the entry is viewed via HTTP.

When generating URLs to be processed by Paperclip (as in the `PollZencoderJob`) we need to supply a `default_protocol` since Paperclip cannot handle protocol relative URLs (see thoughtbot/paperclip#1627). Also in HLS playlists and SMIL files processed by CDNs we default to using the HTTP protocol if non is configured.