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/Utils.fs

@@ -79,7 +79,7 @@ module Migration =
     open Microsoft.Extensions.Logging
 
     /// <summary>The current database version</summary>
-    let currentDbVersion = "v2.2"
+    let currentDbVersion = "v3"
 
     /// <summary>Log a migration step</summary>
     /// <param name="log">The logger to which the message should be logged</param>