Add Remediation Steps for each event to Alerts page

[rich-graylog]

Relates to Graylog2/support#47

What?

Graylog 6.0 introduced Remediation steps to Event Definitions, but in the frontend this is only displayed in the Security perspective, and not the General perspective. Given that this is an open feature for Event Definitions, it should be displayed in the General perspective as well.

Why?

Open/Enterprise users can define Remediation Steps in Event Definitions, but are not able to view them when an event fires (this is only available for users with a Security license currently)

This will likely need to be an expandable/collapsible section within the details of an alert that has fired, but should work with UI design to come up with a solution for how it should be displayed on the page.

[coffee-squirrel]

Related to our case 486, it seems. I didn't realize this was an Open feature, since it was only mentioned in the Enterprise/Security changelog (and the event definitions documentation).

I'd ask that this be extended to enable including remediation steps in notification templates. Specifically: it'd be nice to have one or more new metadata fields for the remediation steps, with plaintext notifications having the raw markdown and HTML notifications having the HTML representation of the markdown.