Publish Organisations to the Publishing API

As part of the work of moving other Applications to rely on the Content
Store we need access to Organisations. This work sends the Organisation
to the Publishing API after save which kicks off a worker and queues it
up to send to the publishing API.

This commit adds the content_id to the Organisation table. This is
generated and used by the content store as a UUID. It generates this on
a before_validation callback. There's also a data_migration to populate
the content_id for all existing Orgs.

Work was based off the work in 63e4f7c. Ticket:
https://trello.com/c/MUqfVwLp/358-getting-orgs-from-whitehall-into-content-store
Publishing guidance changes

The new content publishing guidance manuals are now live. These changes
point to the new guidance on style and how to publish to GOV.UK.
Merge pull request #1773 from alphagov/publish-orgs-to-publishing-api

Publish Organisations to the Publishing API - Tested locally. Correctly publishes to Content Store.
Point directly to plain English section

Point directly to plain English section of the publishing guidance
Fix add_content_id_to_orgs migration

With the migration added in ba5a421 there was both a `null:false` and
index on the new `content_id` column. This prevented the migration from
running as when it came to add the index all the rows in the column were
blank strings.

This commit changes the existing migration to add the `content_id`
column without the null constraint and updates the data migration to
look for Orgs with nil `content_id` fields instead of empty strings.
Add two organisation screenshots.
Merge pull request #1779 from alphagov/publishing-guidance-changes

Publishing guidance changes
Merge pull request #1778 from alphagov/fix-content-id-migration

Fix add_content_id_to_orgs migration
Merge pull request #1780 from alphagov/add_screenshots

Add two organisation screenshots.
Fix SSL issue with phantomjs loading assets.

phantomjs defaults to using SSLv3, which has now been disabled
everywhere.  The protocol used can be changed with an option:

> --ssl-protocol=<val>                 Sets the SSL protocol
>   (supported protocols: 'SSLv3' (default), 'SSLv2', 'TLSv1', 'any')

This PR makes poltergeist set this option when invoking phantomjs.
Merge pull request #1781 from alphagov/fix_phantomjs_ssl_issue

Fix SSL issue with phantomjs loading assets.