Slack notifications need to leverage the severity we're introducing in RHCLOUD-41535.

Acceptance criteria:

• Templates from the engine module MUST NOT be updated.
• Use the beta email template feature for all changes done from this ticket. Ask glepage@redhat.com or rh-ee-gduval for guidance if needed.
• The default Slack template includes the severity when available (it is optional in the action schema). See the UXD slides from the comment below for guidance (work in progress) about the template.
• The custom Slack templates - which may already include a severity from the tenants payload - are updated to include the severity field from RHCLOUD-41535.
• Unit tests are updated.