Absolute URLs for links appearing in feed

note that this changes the oft-used byline()
Issue 1859: Challenge settings don't catch contradictory dates
Issue 1859: Challenge settings don't catch contradictory dates
Adding changes to subscriptions_controller and subscriptions index.html.erb
Issue 2249: Clarifying errors on Prompt Memes
Adding tests to check prompt meme error notice
Issue 2969: Published Date on works not respecting time zone preference
Issue 2969: Changed how original date is computed.
Issue 3494: Moderators can delete PromptMeme sign-ups
Issue 3494: Now with tests
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