Make invited_users folder_permissions column non-nullable

Now that notifications-admin is always sending through
folder_permissions, the folder_permissions column of the invited_user
table can be made non-nullable. The migration also backfills the column
(to []) to account for existing null values.

migrations/versions/0281_non_null_folder_permissions.py

@@ -0,0 +1,26 @@
+"""
+
+Revision ID: 0281_non_null_folder_permissions
+Revises: 0280_invited_user_folder_perms
+Create Date: 2019-03-20 10:12:24.927129
+
+"""
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+revision = '0281_non_null_folder_permissions'
+down_revision = '0280_invited_user_folder_perms'
+
+
+def upgrade():
+    op.execute("UPDATE invited_users SET folder_permissions = '[]' WHERE folder_permissions IS null")
+    op.alter_column('invited_users', 'folder_permissions',
+               existing_type=postgresql.JSONB(astext_type=sa.Text()),
+               nullable=False)
+
+
+def downgrade():
+    op.alter_column('invited_users', 'folder_permissions',
+               existing_type=postgresql.JSONB(astext_type=sa.Text()),
+               nullable=True)