Fixup indexingtask refactor
Don't provide defaults for IndexingTask options

Instead of building a default Indexer and ExternalLinkRegisterer just
don't run anything if they aren't provided.

This allows us to keep the behaviour of the rummager:index rake task the
same and swap out for a new rake task when we're happy it's behaving
properly.
Add deploy_links rake task

Adds links to the rummager index and the external link tracker and
removes old links from the rummager index.