[PM-1011] LaunchDarkly service (#2726)

* LaunchDarkly service

* Load file-based flag values, properly support offline only when self-host

* Simplify tests

* Use interface for LD settings

* Remove tests that will provide inconsistent results depending on machine setup and file fallback

* Fall back to offline mode more actively

* Drive fallback file path with setting

src/Core/Settings/IGlobalSettings.cs

@@ -17,4 +17,5 @@ public interface IGlobalSettings
     ILogLevelSettings MinLogLevel { get; set; }
     IPasswordlessAuthSettings PasswordlessAuth { get; set; }
     IDomainVerificationSettings DomainVerification { get; set; }
+    ILaunchDarklySettings LaunchDarkly { get; set; }
 }