Use server_default for pinned article migration

Theoretically this should work, since default wasn't working. Other
options include making the column nullable, setting a value, then making
it not nullable. This is kind of obnoxious really.

migrations/versions/28e9e049850c_add_pinned_articles.py

@@ -14,8 +14,8 @@
 import sqlalchemy as sa
 
 def upgrade():
-    op.add_column("article", sa.Column("pinned_article", sa.Boolean(), nullable=False, default=False))
-    op.add_column("article_revision", sa.Column("pinned_article", sa.Boolean(), nullable=False, default=False))
+    op.add_column("article", sa.Column("pinned_article", sa.Boolean(), nullable=False, server_default=False))
+    op.add_column("article_revision", sa.Column("pinned_article", sa.Boolean(), nullable=False, server_default=False))
     pass