Implement a MTGO/.dec parser

This new parser can read a decklist given in the Magic Online format, which should allow players to directly copy paste their list from popular deckbuilding sites.

articles/tests/test_template_tags.py

@@ -56,8 +56,7 @@ class ArticleRenderingTestCase(TestCase):
     def test_decklist(self):
         DecklistFactory(
             id="ff521f2e-085c-4cc0-901b-600ec9a71dab",
-            mainboard="4 Daze",
-            sideboard="4 Fry",
+            content="4 Daze\n\n4 Fry",
         )
         article = f"""
         [[https://{SWISS_DOMAIN}/decklists/ff521f2e-085c-4cc0-901b-600ec9a71dab/]]