Merge branch 'file_redirection_cleanup_branch' into 'nightly'

Proper URL redirection fix

This greatly reduces the number of times GNU social looks up the same URL with File_redirection::where().

Someone must have mixed up the exlib's internal setting "follow_redirects" with our own.

This also makes away with the weird backwards recursive handling of protected URLs in File_redirection::lookupWhere(), by saving the chain of redirects the first time HTTPClient:send() looks them up.

See merge request !95