Warn about identical slugs

www.agileplannerapp.com/boards/173808/cards/5403

As a department editor
When creating a document in Whitehall Publisher
I want be warned of possible title duplicates
So that I can create unique titles and avoid
ugly --2, --3 slugs
Tweaks for clarity
Indicate whether the view displays footer metadata
Ensure scrolling navigation doesn't overlap footer meta
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.
Tidy up temporary delegation of body attribute

We want to still fall back to the body attribute of the HtmlAttachment model
whilst the GovspeakContent instance is being created during the data migration.
This also renames the method by which precomputed HTML is accessed with.
Pre-compute govspeak headers

The govspeak headers take almost as long to generate as the body HTML itself as
the entire govspeak has to be parsed to generate them.
Rename html attribute for consistency
Ensure partials can be found in worker

This isn't ideal, but because the govspeak helpers are in whitehall need access
to partials to embed contacts, we need to modify the helpers proxy returned by
ApplicationController such that it knows where to look for the partials.
Add new detailed guidance category
Correct typo
Merge pull request #1750 from alphagov/fix_history_anchor

Fix history anchoring where no footer metadata exists
Move manually_numbered_headings to GovspeakContent

We need to re-compute the HTML when the numbering scheme changes, otherwise
we'll be serving stale HTML.
Mark old columns as deprecated

This will prevent ActiveRecord from throwing any hissy fits when they are
eventually dropped due to it trying to update/set non-existent columns.