Add precompiled_letter service permission

- migration script to add precompiled_letter in service_permission_types table and in service permission types list in models.py

migrations/versions/0167_add_precomp_letter_svc_perm.py

@@ -0,0 +1,24 @@
+"""empty message
+
+Revision ID: 0167_add_precomp_letter_svc_perm
+Revises: 0166_add_org_user_stuff
+Create Date: 2018-02-21 12:05:00
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = '0167_add_precomp_letter_svc_perm'
+down_revision = '0166_add_org_user_stuff'
+
+from alembic import op
+
+
+def upgrade():
+    op.get_bind()
+    op.execute("insert into service_permission_types values('precompiled_letter')")
+
+
+def downgrade():
+    op.get_bind()
+    op.execute("delete from service_permissions where permission = 'precompiled_letter'")
+    op.execute("delete from service_permission_types where name = 'precompiled_letter'")