Refs FedStoa#108 - Add storage for conditional posting via faves

I don't know the correct way to reserve a migration ID, so I've named
this `abcdef012345`. Guidance welcome.

Also I expect to rename the existing `conditional_posting` storage,
because it will be confusingly named if this new conditional posting
method is added.

migrations/versions/abcdef012345_conditional_posting_with_faves.py

@@ -0,0 +1,29 @@
+"""empty message
+
+Revision ID: abcdef012345
+Revises: 52a6ff8551e1
+Create Date: 2019-03-10 20:44:12.345678
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision = 'abcdef012345'
+down_revision = '52a6ff8551e1'
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    ## Rename existing conditional_posting column for clarity.
+    # op.alter_column('settings', 'conditional_posting', new_column_name='conditional_posting_hashtags')
+    op.add_column('settings', sa.Column('conditional_posting_faves', sa.Boolean(), nullable=False))
+
+
+def downgrade():
+    ## Should be dropped in dc37a95190f6, provided it's correctly renamed back.
+    # op.alter_column('settings', 'conditional_posting_hashtags', new_column_name='conditional_posting')
+    op.drop_column('settings', 'conditional_posting_faves')
+