Bump govuk_content_models to 8.1.0

This adds a new tag type for organisations.
Add field to artefact form to tag organisations

This adds a new field to the "tags" section of the artefact form, in 
similar style to other tags fields, to allow zero or more organisations 
to be selected for the artefact.
Bump Plek to 1.7.0

This allows us to provide an alternate URLs for services which this 
app depends on.
Add an organisation importer

This adds a new class OrganisationImporter. It requests the list of all 
organisations from the API, and iterates over each result. If a tag 
doesn't already exist for the organisation, one will be created. If a 
tag already exists, then the tag title will be updated only if the 
organisation name has changed.

The importer is invoked through a new Rake task: organisations:import
Set up the Whenever gem

To keep the list of organisations up-to-date, we should run the importer 
on a regular basis. Our convention is to use the Whenever gem to set up 
app-based cron jobs.

The task definitions will live in alphagov-deployment, but this commit 
sets up Whenever, along with a placeholder config in schedule.rb.
Sort tags in alphabetical order based on title
Add section to README about importing org tags

This also tidies up the Markdown a bit.
Notify Airbrake when saving an org fails

Change the importer to notify the Airbrake gem when an org cannot be 
created or updated due to a validation error. This also increases the 
severity of log messages for these errors to 'warn'.
Merge pull request #136 from alphagov/tag-content-with-organisations

Allow artefacts to be tagged with organisations