Add migration for user_folder_permissions table

Changed the user_to_service mapping table into a model called
ServiceUser. When looking at users who have permission for a folder
we are only interested in users for a particular service, not all users,
so we can use the ServiceUser model to access folder permissions.

Added a user_folder_permissions table which contains the service_id,
user_id and template_folder_id. There are links between
user_folder_permissions and TemplateFolder, and between
user_folder_permissions and ServiceUser.

migrations/versions/0266_user_folder_perms_table.py

@@ -0,0 +1,32 @@
+"""
+
+Revision ID: 0266_user_folder_perms_table
+Revises: 0265_add_confirm_edit_templates
+Create Date: 2019-02-26 17:00:13.247321
+
+"""
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects import postgresql
+
+revision = '0266_user_folder_perms_table'
+down_revision = '0265_add_confirm_edit_templates'
+
+
+def upgrade():
+    op.create_unique_constraint('ix_id_service_id', 'template_folder', ['id', 'service_id'])
+    op.create_table('user_folder_permissions',
+        sa.Column('user_id', postgresql.UUID(as_uuid=True), nullable=False),
+        sa.Column('template_folder_id', postgresql.UUID(as_uuid=True), nullable=False),
+        sa.Column('service_id', postgresql.UUID(as_uuid=True), nullable=False),
+        sa.ForeignKeyConstraint(['template_folder_id', 'service_id'], ['template_folder.id', 'template_folder.service_id'], ),
+        sa.ForeignKeyConstraint(['user_id', 'service_id'], ['user_to_service.user_id', 'user_to_service.service_id'], ),
+        sa.ForeignKeyConstraint(['template_folder_id'], ['template_folder.id'], ),
+        sa.PrimaryKeyConstraint('user_id', 'template_folder_id', 'service_id'),
+    )
+
+
+
+def downgrade():
+    op.drop_table('user_folder_permissions')
+    op.drop_constraint('ix_id_service_id', 'template_folder', type_='unique')