refactor: pass DbContextOptions into context directly

turns out efcore doesn't like it when we create a new options instance
(which includes a new data source *and* a new logger factory)
every single time we create a context. this commit extracts
OnConfiguring into static methods which are called when the context is
added to the service collection and when it's manually created for
migrations and the importer.

Foxnouns.Backend/Services/FediverseAuthService.Mastodon.cs

@@ -2,7 +2,6 @@ using System.Net;
 using System.Web;
 using Foxnouns.Backend.Database;
 using Foxnouns.Backend.Database.Models;
-using Minio.DataModel.ILM;
 using Duration = NodaTime.Duration;
 using J = System.Text.Json.Serialization.JsonPropertyNameAttribute;