[#1001] Allow routing to redirect to HTTPS on userspace as well

* Only registering prefer_ssl on uploaded media files (so it's not a
  journal-wide thing)

* pass in is_ssl to DW::Routing->call on journalspace urls
[#1001] Generate HTTPS links for media files when we're on an HTTPS page

* some refactoring so that LJ::User->journal_base takes a hash of opts
  for a second argument

* pass in current $LJ::IS_SSL status (important because we don't want
  HTTPS on for all links to journals, just specific ones for now)

Fixes #1001.