refactor `SlackMessage.channel_id` (`CHAR` field) to `SlackMessage.channel` (foreign key relationship) #2291

Job	Run time
Linting and tests / Lint database migrations	1m 10s
Linting and tests / Test technical documentation	13s
Linting and tests / Lint, test, and build frontend	1m 40s
Linting and tests / Helm Chart Unit Tests	20s
Linting and tests / Lint entire project	1m 10s
Linting and tests / Ensure database migrations drop columns the correct way	9s
Linting and tests / Backend Tests: MySQL + RabbitMQ (RBAC enabled: True)	3m 55s
Linting and tests / mypy	1m 27s
Linting and tests / Backend Tests: MySQL + RabbitMQ (RBAC enabled: False)	3m 55s
Linting and tests / Backend Tests: Plugin	33s
Linting and tests / Backend Tests: PostgreSQL + RabbitMQ (RBAC enabled: True)	3m 21s
Linting and tests / Backend Tests: SQLite + Redis (RBAC enabled: True)	3m 17s
Linting and tests / Unit tests - Migrators	22s
Linting and tests / Backend Tests: SQLite + Redis (RBAC enabled: False)	3m 2s
Linting and tests / Backend Tests: PostgreSQL + RabbitMQ (RBAC enabled: False)	3m 55s
Snyk security scan / Snyk security scan	1m 21s
Linting and tests / Standard e2e tests (10.3.0) / Grafana: 10.3.0	8m 28s
Linting and tests / Standard e2e tests (11.2.0) / Grafana: 11.2.0	8m 0s
Linting and tests / Standard e2e tests (latest) / Grafana: latest	8m 41s
	54m 59s

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