Move OpenGraph property generation to models (#52)

- Add auto-OpenGraph field to web log
- Only generate properties for posts/pages without them if this flag is set
- Set flag to yes on v3 database migration
- Add JSON converter for OpenGraph type
- Add tests for models

src/MyWebLog.Data/Converters.fs

@@ -65,6 +65,14 @@ module Json =
         override _.ReadJson(reader: JsonReader, _: Type, _: MarkupText, _: bool, _: JsonSerializer) =
             (string >> MarkupText.Parse) reader.Value
 
+    /// <summary>Converter for the <see cref="OpenGraphType" /> type</summary>
+    type OpenGraphTypeConverter() =
+        inherit JsonConverter<OpenGraphType>()
+        override _.WriteJson(writer: JsonWriter, value: OpenGraphType, _: JsonSerializer) =
+            writer.WriteValue(string value)
+        override _.ReadJson(reader: JsonReader, _: Type, _: OpenGraphType, _: bool, _: JsonSerializer) =
+            (string >> OpenGraphType.Parse) reader.Value
+
     /// <summary>Converter for the <see cref="Permalink" /> type</summary>
     type PermalinkConverter() =
         inherit JsonConverter<Permalink>()
@@ -159,6 +167,7 @@ module Json =
           CustomFeedSourceConverter()
           ExplicitRatingConverter()
           MarkupTextConverter()
+          OpenGraphTypeConverter()
           PermalinkConverter()
           PageIdConverter()
           PodcastMediumConverter()