feat: add Slack OAuth integration and connector routes

- Introduced Slack OAuth support with new environment variables for client ID, client secret, and redirect URI.
- Implemented Slack connector routes for OAuth flow, including authorization and callback handling.
- Updated configuration to support both new OAuth format and legacy token handling.
- Enhanced the Slack indexer to decrypt tokens when necessary, ensuring compatibility with existing encrypted credentials.
- Removed outdated Slack connector UI components and adjusted frontend logic to reflect the new integration.

surfsense_backend/app/routes/__init__.py

@@ -26,6 +26,7 @@ from .podcasts_routes import router as podcasts_router
 from .rbac_routes import router as rbac_router
 from .search_source_connectors_routes import router as search_source_connectors_router
 from .search_spaces_routes import router as search_spaces_router
+from .slack_add_connector_route import router as slack_add_connector_router
 
 router = APIRouter()
 
@@ -44,6 +45,7 @@ router.include_router(airtable_add_connector_router)
 router.include_router(linear_add_connector_router)
 router.include_router(luma_add_connector_router)
 router.include_router(notion_add_connector_router)
+router.include_router(slack_add_connector_router)
 router.include_router(new_llm_config_router)  # LLM configs with prompt configuration
 router.include_router(logs_router)
 router.include_router(circleback_webhook_router)  # Circleback meeting webhooks