Move MarkdownPost to its own file

lib/DB.rakumod

@@ -8,18 +8,7 @@ use JSON::Class:auth<zef:vrurg>;
 
 use DB::Post;
 use DB::BlogMeta;
-
-#| A plain markdown post
-class MarkdownPost does Post is json {
-    #| Marker for disambiguation between post types in json representation, the
-    #| cheaty way
-    has Bool:D $.markdown = True;
-
-    method title(--> Str:D) {
-        markdown-title($!source)
-    }
-}
-
+use DB::MarkdownPost;
 
 #| A plain markdown post
 class IdrisPost does Post is json {