[WIP] Modify schema, creation of new attributes

Added some new attributes changes in the Member and Mailing List model
This Merge Request is work-in-progress

• Now I am following Mark's Implementation, see here,here and here for Mark's implementation.

src/mailman/database/alembic/versions/58858f75303c_added_new_bounce_attributes.py

@@ -10,7 +10,7 @@ Create Date: 2019-06-28 01:10:48.210528
 import sqlalchemy as sa
 
 from alembic import op
-from mailman.database.helpers import exists_in_db
+from mailman.database.helpers import exists_in_db, is_sqlite
 
 # revision identifiers, used by Alembic.
 revision = '58858f75303c'
@@ -22,15 +22,11 @@ def upgrade():
     if not exists_in_db(op.get_bind(), 'address', 'bounce_info'):
         op.add_column('address', sa.Column('bounce_info', sa.PickleType(),
                                            nullable=True))
-    with op.batch_alter_table('domain') as batch_op:
-        batch_op.create_unique_constraint('uq_domain_mailhost', ['mail_host'])
     # ### end Alembic commands ###
 
 
 def downgrade():
     # ### commands auto generated by Alembic - please adjust! ###
-    with op.batch_alter_table('domain') as batch_op:
-        batch_op.drop_constraint('uq_domain_mailhost', type_='unique')
-    with op.batch_alter_table('address') as batch_op:
-        batch_op.drop_column('bounce_info')
+    if not is_sqlite(op.get_bind()):
+        op.drop_column('address', 'bounce_info')
     # ### end Alembic commands ###