Topic: add original_url column

There are probably better ways to do this, but this should be a simple
way to start working towards some link canonicalization.

tildes/alembic/versions/6ede05a0ea23_topic_add_original_url_column.py

@@ -0,0 +1,30 @@
+"""Topic: add original_url column
+
+Revision ID: 6ede05a0ea23
+Revises: 09cfb27cc90e
+Create Date: 2018-09-12 18:45:44.768561
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+from tildes.models.topic import Topic
+
+
+# revision identifiers, used by Alembic.
+revision = "6ede05a0ea23"
+down_revision = "09cfb27cc90e"
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.add_column("topics", sa.Column("original_url", sa.Text(), nullable=True))
+
+    session = sa.orm.Session(bind=op.get_bind())
+    session.query(Topic).update({"original_url": Topic.link}, synchronize_session=False)
+    session.commit()
+
+
+def downgrade():
+    op.drop_column("topics", "original_url")

tildes/tildes/models/topic/topic.py

@@ -106,6 +106,7 @@ class Topic(DatabaseModel):
     _markdown: Optional[str] = deferred(Column("markdown", Text))
     rendered_html: Optional[str] = Column(Text)
     link: Optional[str] = Column(Text)
+    original_url: Optional[str] = Column(Text)
     content_metadata: Dict[str, Any] = Column(
         MutableDict.as_mutable(JSONB(none_as_null=True))
     )
@@ -207,6 +208,7 @@ class Topic(DatabaseModel):
         new_topic = cls._create_base_topic(group, author, title)
         new_topic.topic_type = TopicType.LINK
         new_topic.link = link
+        new_topic.original_url = link
 
         incr_counter("topics", type="link")