Update migrations

[VishakhaSainani-Josh]

Update migrations-

1. Add Column contributors url in repositories table
2. . Allow null values for contribution id in transaction table. As transaction contains both redeemed and gained transactions so contribution id will be null for redeemed one.
3. Need to add column github_event_id in contributions table to use the gh archive contribution id for querying instead of usernames.
4. Set Not null constraint for contributors_url in repositories table
5. Set Not null constraint for github_event_id in contributions table.

Please review all migration related changes in this PR

[Suhaan-Bhandary-Josh]

Could you point this branch to it's parent, would be easier to review