update db schema to included last updated on for each table

[falkben]

Related to #39 we want a way to track when data in the database has been updated.

for an "updated_on" column, alembic won't auto-generated data for existing rows. We need to create a migration manually to support adding columns that have a default value.

One suggestion i've seen is that inside the migration, copy the data to a separate table with a new name, then copy it back into the original named table, with the updated column.