Add editable field for briefing redirection url

[tomharrigan]

Allow the default home URL link to be overridden with a custom link in the redirectionUrl field of the JSON output.

https://developer.amazon.com/docs/flashbriefing/flash-briefing-skill-api-feed-reference.html#feed-format-details

Todo:
-Add editable URL field on briefing post type
-When generating output, check for existence of a URL in this field, if exists, output; otherwise output home url of the site.