Add AppSettings capability

[JeremyCaney]

Create an OnTopic AppSettings configuration store that can be used to store global settings such as (non-secret) API keys or site-wide preferences.

Current Approach

Today, customers store application settings in a variety of places:

• config files (such as web.config or appsettings.config)
• secrets.json files (in development) and Azure App Service Settings (in production)
• Individual Topic classes, if it's not a secret (see below)
• Startup, if it's not a secret

Some sites even use a combination of each of these.

Topic-Based Approach

Given the nature of OnTopic as a semi-structured data source, it makes sense for OnTopic sites to be configured via Topics. Today, this is done on a per Topic basic, typically as a subclass of Page. That's fine for one-off pages, such as a /Search page storing an API key, but it's impractical for values that need to be shared across multiple pages, as configuration settings are only available in that context.

Ideal Approach

Ideally, we'll instead approach this through something like the following:

• ContentTypeDescriptor: AppSettings (LookupList)
  • TopicList: Settings of AppSettingsItem (LookupListItem)
  • AllowedContentTypes: AppSettings (thus allowing namespacing)
• Root:Configuration:AppSettings as root store
• TopicAppSettings (IConfiguration) class for storing, retrieving settings
• TopicConfiguration class for mapping TopicAppSettings to a value object and passing it to e.g. AddTopicSupport().

This way, any custom application setting can easily be added to OnTopic without needing to create a custom ContentTypeDescriptor, and those application settings can easily be namespaced in case a particular context (such as /Search) has multiple settings.