[feature request] Include silence alarm link in the default email templates

Similarly to https://github.com/prometheus/alertmanager/issues/722, it would be very helpful to include a link directly to the filled out silence page in Alertmanager's UI. An issue that commonly plagues our team is that when you have an alarm flapping at 3-o-clock in the morning, and you know it can be safely ignored, it is often very difficult to find the correct label/value pairs to set in order to silence the alarm after it has already resolved. Having these links preserved in the email body of the original alert would alleviate this.

We have successfully added these links to our slack alerts using the methods outlined in this blog: https://hodovi.cc/blog/creating-awesome-alertmanager-templates-for-slack/
But I've been struggling to get the same thing to work in the email templates (escaping URL encoded strings is hard in the golang html/template package?). If I can get this working in our set up I'll open a PR for it, but maybe someone has already done this and can point us in the correct direction.

Start by reviewing Alertmanager's default email templates and how Go's html/template package handles URL-encoded strings. Compare the requested silence link with the existing Slack-template approach and the related issue 722. Done means the default email body preserves a usable link to the filled-out silence page.