Dependency Injection migration

[JeremyCaney]

The sample application (and most implementors) currently wire up services manually via a custom "activator" that constructs each dependency via a switch, rather than using .NET's standard, built-in dependency-injection container. This replaces that with idiomatic DI registration.

Tasks

• Add an AddOnTopic() registration helper for the core mapping/lookup services (ServiceCollectionExtensions, OnTopic.AspNetCore.Mvc)
• Add an AddTopicNavigation<TViewModel>() helper for the hierarchical navigation service (ServiceCollectionExtensions)
• Add an AddSqlTopicRepository() helper for the SQL repository (OnTopic.Data.Sql)
• Add an AddTopicCaching() helper for the caching repository (OnTopic.Data.Caching)
  • Must register ITopicRepository as a singleton; anything less silently creates a fresh, empty cache per request
• Delete the activator from the sample host; wire it up via the new registration helpers (SampleActivator, Program, OnTopic.AspNetCore.Mvc.Host)
• Delete the activator from the integration test host; wire it up the same way (SampleActivator, OnTopic.AspNetCore.Mvc.IntegrationTests.Host)
• Apply the same pattern to the OnTopic Editor's SampleActivator
• Create unit tests to validate the above logic