feat(slack): add alerts-only integration mode

Adds an integration mode setting to the Slack integration. In the new
alerts_only mode, conversations continue to sync to the connected Slack
channel, but messages sent in Slack threads are never delivered to the
customer, so teams can use the channel for alerts and internal
discussion. Existing hooks are backfilled to two_way, keeping current
behavior unchanged.

Also fixes channel selection replacing hook settings wholesale, which
would have wiped the stored integration mode.

app/javascript/dashboard/api/integrations.js

@@ -29,6 +29,13 @@ class IntegrationsAPI extends ApiClient {
     return axios.post(`${this.baseUrl()}/integrations/hooks`, hookData);
   }
 
+  updateHook(hookId, hookData) {
+    return axios.patch(
+      `${this.baseUrl()}/integrations/hooks/${hookId}`,
+      hookData
+    );
+  }
+
   deleteHook(hookId) {
     return axios.delete(`${this.baseUrl()}/integrations/hooks/${hookId}`);
   }