fix(dashboards): Add unique constraint for user and dashboard pair

[narsaynorath]

In an attempt to change the syntax for defining the uniqueness constraint, the original constraint was dropped and only updated the Django state.

This reintroduces the constraint so we actually enforce it in the DB on inserts.

[github-actions]

This PR has a migration; here is the generated SQL for src/sentry/migrations/0927_dashboard_add_unique_constraint_user_dashboard.py

for 0927_dashboard_add_unique_constraint_user_dashboard in sentry

--
-- Custom state/database change combination
--
CREATE UNIQUE INDEX CONCURRENTLY "sentry_dashboardfavoriteuser_user_id_dashboard_id_2c7267a5_uniq" ON "sentry_dashboardfavoriteuser" ("user_id", "dashboard_id");
ALTER TABLE "sentry_dashboardfavoriteuser" ADD CONSTRAINT "sentry_dashboardfavoriteuser_user_id_dashboard_id_2c7267a5_uniq" UNIQUE USING INDEX "sentry_dashboardfavoriteuser_user_id_dashboard_id_2c7267a5_uniq";

[narsaynorath]

@wedamija Following up on our conversation in #92272 (comment)

I ran a query to count how many instances could violate the uniqueness constraint in this redash query and it hasn't been violated (i.e. all the counts are 1). This is my PR to update the DB to contain the constraint, since the state is pointing to sentry_dashboardfavoriteuser_user_id_dashboard_id_2c7267a5_uniq as the name, I also made the DB change use the same name