Allow organisations in subscriptions
Allow organisations in subscriptions

This was removed as part of redirecting organisation subscriptions to email
alert frontend., however the organisation property is still used in other
subscriptions like:

https://www.gov.uk/government/email-signup/new?email_signup%5Bfeed%5D=https%3A%2F%2Fwww.gov.uk%2Fgovernment%2Fpublications.atom%3Fdepartments%255B%255D%3Deducation-and-skills-funding-agency%26publication_filter_option%3Dcorrespondence

These paths are currently erroring:
https://sentry.io/organizations/govuk/issues/1169351678/?project=202259&query=url%3A%22https%3A%2F%2Fwww.gov.uk%2Fgovernment%2Femail-signup%22&statsPeriod=14d

with an error like:

```
UrlToSubscriberListCriteria::StaticData::UnknownStaticDataKey
organisations
.../lib/url_to_subscriber_list_criteria.rb:89:in `content_id'
.../lib/url_to_subscriber_list_criteria.rb:75:in `lookup_content_id'
.../lib/url_to_subscriber_list_criteria.rb:50:in `block (2 levels) in map_url_to_hash'
.../lib/url_to_subscriber_list_criteria.rb:50:in `map'
.../lib/url_to_subscriber_list_criteria.rb:50:in `block in map_url_to_hash'
.../lib/url_to_subscriber_list_criteria.rb:46:in `each'
.../lib/url_to_subscriber_list_criteria.rb:46:in `each_with_object'
.../lib/url_to_subscriber_list_criteria.rb:46:in `map_url_to_hash'
.../lib/url_to_subscriber_list_criteria.rb:14:in `convert'
.../app/models/email_signup.rb:27:in `criteria'
.../app/models/email_signup.rb:23:in `ensure_subscriber_list_exists'
.../app/models/email_signup.rb:16:in `save'
.../app/controllers/email_signups_controller.rb:15:in `create'
```

I think if we add organisation back here, then the remaining things will continue
to work until we retire them too.

alphagov/whitehall,alphagov/whitehall,alphagov/whitehall,alphagov/whitehall