Keep sidekiq knowledge inside ScheduledPublisher

Create `enqueue` and `queue_size` methods to achieve this, which has
made things a little more DRY.

This also fixes a potential future issue in cancel_scheduled_publishing
where it doesn't filter jobs to those belonging to the relevant worker.
This isn't an issue at present because currently there is only the one
worker.
Improve task to rebuild scheduled publishing queue

This currently cancels and rescheduled all editions due for publishing.
This won't catch any jobs scheduled for editions that are no longer due
to be published (ie orphaned sidekiq jobs).

Instead, this now clears all ScheduledPublisher jobs, and then
re-enqueues all scheduled editions.
Merge pull request #262 from alphagov/scheduled_publishing_resync

 Improve task to rebuild scheduled publishing queue
Send published documents to the content store.

Does not include the full content of the documents
as this is only for the purpose of emailing topic
updates.

I've made an assumption here that editions where
fact check was skipped should be considered minor
updates.

Future changes as part of this work:
- Adding topic/browse page associations

Future changes not part of this work:
- Migrating to full content store tag format
- Adding full body of content
- Fully implement content store functionality,
  removing "placeholder" status
Consider all changes de facto major.

This is following discussion with @JordanHatch and
@alext on https://github.com/alphagov/publisher/pull/263
wherein we decided to make all changes major for
this publisher (as we do for `collections-publisher`)
until such a time as we can add a `whitehall`-style
major/minor change interface.
Merge pull request #263 from alphagov/send-documents-to-content-store

Send published documents to the content store.

https://www.pivotaltracker.com/story/show/79283750