Update labels on MHRA's Medical safety alerts finder
Update labels on MHRA's Medical safety alerts finder

Changes:
 - facet name from 'Alert type' to 'Message type'
 - document noun (used for pagination) from 'alert' to 'message'

https://govuk.zendesk.com/agent/tickets/4893256

alphagov/specialist-publisher,alphagov/specialist-publisher,alphagov/specialist-publisher