Update Swagger configuration

[corygehr]

• Adds SwaggerUI to replace API Management portal
• Adds SwaggerHosts configuration element and honors this on SwaggerDoc generation
  • Must be set properly on production hosts, usually to Front Door URL
• Updates name of CosmosCovidSafeSchemaConfiguration to CosmosSchemaConfigurationSection
  • Clarifies class purpose and source
• Fixes comments on some endpoints
• Hides HEAD method from API docs for MessageController