add created timestamp to tokens table

[chris48s]

One of the things that has come out of https://github.com/badges/shields-ops/issues/38 is: It would be quite useful to know how long we've held a given token for in a more explicit way then relying on the id sequence. In some ways I'm a bit late to the party, but better late than never.

This PR creates a migration that adds a column called created to the github_user_tokens table. We'll need to manually run this migration on production after deploying it.

The created column defaults to CURRENT_TIMESTAMP which means:

• When we apply the migration it will set the created timestamp to now when we apply the migration for any existing records
• The existing code can run with or without this migration applied
• When we add new tokens they'll automatically be timestamped at INSERT time with no code changes

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[chris48s]

migration is now applied in prod