Add issued date to medical safety alerts

Add the issued date attribute to the medical safety alert date metadata
in the model so that it can be shown in the frontend. This corresponds
to this commit in specialist publisher:
alphagov/specialist-publisher#281

The trello ticket can be found here:
https://trello.com/c/QFE3nYlc/361-add-issued-date-to-medical-safety-alerts-2
Merge pull request #94 from alphagov/feature/add-issued-date-to-medical-safety-alerts

Add issued date to medical safety alerts
Merge pull request #95 from alphagov/fetch-from-content-store

Fetch from content store
Refactor Finders to pull from content_store

This commit contains the work needed to pull Finders from the
content_store and removes the ability for Finders to be retrieved from
the content-api.

Finders and email-alert-subs now pull their content_item and the
relevant tests have been updated to stub the content_store.
Email signup links for content stored Finders

In order to for users to signup for alerts from Finders, we need to
display a link to the signup page. This commit parses the response from
the content store and pulls out the finder_email_signup field in the
Finder initialization if it's present. The nil check happens in the
view, so we assign it as a nil value if the field isn't in the response.

This commit also adds the mail icon (along with a retina version) and
the CSS required to display this.