Add new model for delegating govspeak content to

This is the first step towards storing pre-computed HTML from govspeak. For now,
we've just moved the body over to the delegated association. Validations are
skipped in the data migration to skip the govspeak safety checks, otherwise the
data migration takes an age.
Update attachment cloning to handle delegate body

This insures both the HtmlAttachment and it's GovspeakContent association are
correctly copied when editions are re-editioned.
Rename methods for clarify
Make tests less stubby
Prevent Sidekiq jobs from lingering between tests
Update govspeak to allow spans in safe HTML

govspeak has been updated to allow span elements to pass HTML safety validation.
This is required so that the computed HTML can be saved on GovspeakContent
without numbered headings (which use spans) causing it to fail.
Pre-compute govspeak HTML in background job.

Every time the body on a GovspeakContent instance changes, we want to re-compute
the computed_html. This relies on ActiveRecord dirty tracking to determine when
a job needs to be queued.