Message Permlinks within Alert Notifications

## Proposed Functionality
It would be great to be able to include the Permlink to messages within the Alert Notifications.
If the Permlink was made available in the same way as `message.source` and the other values, that would work well with the current implementation of iterating over messages in the backlog in order to include that data within the alert message.

## Current Behavior
Currently unable to link to messages which have triggered alerts. As far as I am aware, we are currently only able to include a link to the stream which the triggering message is in.

## Possible Solution
Make the Permlink value of a message available through a similar method as other message content.

## Work Around
This workaround has been tested on:
- graylog-server 3.1.2-1
- graylog-server 3.2.1-1

Including the below inside the backlog iteration of your notification email should provide a functioning link to the message(s) which triggered the alert.

`http:///messages/${message.index}/${message.id}`

__Warning: This has only been tested on messages stored in a single index.__

## Context
Being able to receive an alert, view the details in the email and provide a link to the exact message that has caused the alert can increase response and resolution time to alerts.

## Your Environment

* Graylog Version: 3.1.2
* Elasticsearch Version: 6.8.4
* MongoDB Version: 4.0.13
* Operating System: Ubuntu Server 18.04
* Browser version: N/A