Add migration to initialize Person weights

That's for Persons that existed before the weights were added to model.

budget/migrations/versions/f629c8ef4ab0_initialize_all_members_weights_to_1.py

@@ -0,0 +1,39 @@
+"""Initialize all members weights to 1
+
+Revision ID: f629c8ef4ab0
+Revises: 26d6a218c329
+Create Date: 2016-06-15 09:40:30.400862
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = 'f629c8ef4ab0'
+down_revision = '26d6a218c329'
+
+from alembic import op
+import sqlalchemy as sa
+
+# Snapshot of the person table
+person_helper = sa.Table(
+    'person', sa.MetaData(),
+    sa.Column('id', sa.Integer(), nullable=False),
+    sa.Column('project_id', sa.String(length=64), nullable=True),
+    sa.Column('name', sa.UnicodeText(), nullable=True),
+    sa.Column('activated', sa.Boolean(), nullable=True),
+    sa.Column('weight', sa.Float(), nullable=True),
+    sa.ForeignKeyConstraint(['project_id'], ['project.id'], ),
+    sa.PrimaryKeyConstraint('id')
+)
+
+
+def upgrade():
+    op.execute(
+        person_helper.update()
+        .where(person_helper.c.weight == None)
+        .values(weight=1)
+    )
+
+
+def downgrade():
+    # Downgrade path is not possible, because information has been lost.
+    pass