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 a GovspeakContent instance is saved (created or updated), a job is
queued to update the (re-)computed the HTML. Note that a little dance is
necessary to prevent an endless loop of jobs being queued when the record is
updated with the computed HTML.