Remove old panopticon task

Registering only with panopticon isn't that helpful, and the new
reregister tasks allow registering everything with panopticon and any
other appropriate apps.  Therefore, remove this script to avoid
confusing duplication.
Support sending republishes to content store

Update the `render_for_publishing_api` method to accept a parameter
telling it whether it's a republish event.  If so, it returns an
`update_type` field of `republish`.  This is understood by the content
store, and allows us to re-register content with the content store,
without causing actions like email alerts to happen.
Add task for re-registering content

This adds a PublishedSlugRegisterer class, which performs whatever
registration is necessary, when given a sequence of slugs for published
content.  It logs the actions taken, including producing a listing of
all slugs supplied to it by whether they were successfully processed,
not found, or gave an error when being processed.

Uses logic copied from the old panopticon registration rake task to
retry panopticon registration up to 3 times for an artefact if it times
out.  We only try the registration once with the content store, since we
expect that to be a much more reliable api endpoint.

The new rake tasks are all very simple; obtaining a list of slugs, and
passing them to the registerer.  Several tasks are supported:

  rake reregister:all
    Re-register all published editions

  rake reregister:by_mainstream_browse
    Re-register published editions tagged to a browse page, read from
    the BROWSE_PAGE environment variable

  rake reregister:by_organisation
    Re-register published editions tagged to an organisation, read form
    the ORGANISATION environment variable

  rake reregister:by_topic
    Re-register published editions tagged to a topic, read from the
    TOPIC environment variable

  rake reregister:slugs_from_stdin
    Re-register published editions for slugs read from stdin

I've run these tasks successfully in my dev environment, checking
manually that they update content-store and panopticon (and as a result,
search).
Tidy up some wording in task descriptions
Put the test setup in a setup block

This is more typical use, and ensures the test fixtures get torn down
after the tests.
Use an options hash for render_for_publishing_api

This makes it more obvious what the boolean parameter is for at the
point at which it's called.
Update test name to avoid a duplicate

We need a different test name for the test of formatting a document for
the content API for republishing
Add basic test of PublishedSlugRegisterer
Default to a major update type

After discussion, defaulting to a non-republish updated type was
preferred to requiring this option
Merge pull request #290 from alphagov/add-republish-tasks

Add republish tasks