Suppress webhooks when GitLab is in Silent Mode

What does this MR do and why?

This MR prevents webhooks from executing or being queued when GitLab is in Silent Mode.

• Issue: #393639 (closed)

There is no Changelog trailer because Silent Mode is an experiment.

How to set up and validate locally

1. Create a project hook: 2. Choose a project, and go Settings > Webhooks. 3. Click Add new webhook. 4. Enter a `URL of a webhook receiver. You can visit https://webhook.site to generate a unique webhook receiver URL to test with. 5. Under Trigger, select Issues events. 6. Select Add webhook.
2. Close the issue, you should receive a webhook.
3. Now set the instance to Silent Mode on the Rails console:

   Gitlab::CurrentSettings.update!(silent_mode_enabled: true)

4. Wait up to a minute for it to take effect (see #405433).
5. Reopen the issue, close it again, you should not receive any webhooks.

Remember to disable Silent Mode again afterwards:

Gitlab::CurrentSettings.update!(silent_mode_enabled: false)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #393639 (closed)