[feat] Adding a few DB triggers to delete permissions when items are soft deleted

[kopancek]

These entities are referenced via foreign key in the user_repo_permissions table, and are soft deleted potentially:

• user
• user_external_account
• repo

All of them would leave rows in the user_repo_permissions table, which is not desired. The user_external_account one was the most problematic from the point of view of behavior of authzQuery.

This change only affects the new unified permissions table, old behavior is not affected.

Test plan

Tested locally, directly in the database with the following scenarios:

1. up and down migration works correctly
2. user_repo_permissions table is empty -> nothing happens
3. user_repo_permissions has data from 2 users, 2 repos and 2 external accounts
   1. mark repo as deleted -> only the rows with the same repo_id are deleted
   2. mark user_external_account as deleted -> only the rows with same user_id and user_external_account are deleted
   3. mark user as deleted -> only the rows with the same user_id are deleted

[sashaostrikov]

LGTM!

[mrnugget]

Very cool! Can you also add some high-level "integration-y" test that ensures these triggers work and that fail if someone touches them?

Example: this test here relies on the repo statistics triggers https://github.com/sourcegraph/sourcegraph/blob/8e2430627cdfe42258eeee6bc8873f36c2a478bd/internal/database/repo_statistics_test.go#L18

It uses a DB, inserts data and then checks that the triggers did their work. I think a similar test would also work here.