chore: convert two slack channel ID char fields to foreign keys #2109

Job	Run time
Linting and tests / Lint entire project	1m 21s
Linting and tests / Lint database migrations	1m 1s
Linting and tests / Lint, test, and build frontend	1m 32s
Linting and tests / mypy	1m 27s
Linting and tests / Test technical documentation	18s
Linting and tests / Backend Tests: MySQL + RabbitMQ (RBAC enabled: True)	4m 20s
Linting and tests / Backend Tests: Plugin	33s
Linting and tests / Backend Tests: MySQL + RabbitMQ (RBAC enabled: False)	4m 5s
Linting and tests / Backend Tests: PostgreSQL + RabbitMQ (RBAC enabled: True)	3m 42s
Linting and tests / Backend Tests: SQLite + Redis (RBAC enabled: True)	3m 35s
Linting and tests / Backend Tests: PostgreSQL + RabbitMQ (RBAC enabled: False)	3m 58s
Linting and tests / Backend Tests: SQLite + Redis (RBAC enabled: False)	3m 26s
Linting and tests / Helm Chart Unit Tests	15s
Linting and tests / Unit tests - Migrators	23s
Snyk security scan / Snyk security scan	1m 19s
Linting and tests / Standard e2e tests (11.2.0) / Grafana: 11.2.0	7m 46s
Linting and tests / Standard e2e tests (10.3.0) / Grafana: 10.3.0	7m 51s
	46m 52s

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