refactor: replace latest_checkout table primary key

[gustavobtflores]

This PR modifies checkout_id in latest_checkout to not be a primary key and introduces a surrogate primary key called id as it is required by Django. This is done to avoid that the database throws an exception for duplicated primary key, we are already checking for duplicity with a UNIQUE constraint and the target is not the checkout_id field.

Reasoning

This happens because we do a upsert when inserting in latest_checkout table, we have a query like:

INSERT INTO latest_checkout (
    checkout_id, origin, tree_name,
    git_repository_url, git_repository_branch, start_time
)
VALUES (%s, %s, %s, %s, %s, %s)
ON CONFLICT (origin, tree_name, git_repository_url, git_repository_branch)
DO UPDATE SET
    start_time = EXCLUDED.start_time,
    checkout_id = EXCLUDED.checkout_id
WHERE latest_checkout.start_time < EXCLUDED.start_time

If we try to update a checkout_id that is already in the database, we will get a duplicated primary key exception, where it should just do nothing. We cannot have multiple conflict targets in a single query, so the solution is to just remove it and ensure uniqueness with the UNIQUE constraint.

The UNIQUE constraint cannot be transformed in a composite primary key because it allows NULL values.

Closes #1680

[MarceloRobert]

LGTM