Add a Go-To Action to community emails

## Feature Request

It would be cool to add a Gmail Go-To Action to the community emails that users receive like the image below.


### Implementation

Implementing this would require adding the code given below to the email body.

```html

{
"@context": "http://schema.org",
"@type": "EmailMessage",
"potentialAction": {
"@type": "ViewAction",
"name": "View Discussion",
"url": "https://discuss.flarum.org/d/27394"
},
"description": "Flarum 1.0.0 Released"
}

```

The `url` and `description` would vary.

Source: [Go-To Actions | Email Markup | Google Developers](https://developers.google.com/gmail/markup/reference/go-to-action#view_action)

No file or test entry point is named in the issue. Start by locating the templates or code that assembles community email bodies, then compare the requested JSON-LD with Google's Go-To Actions reference; done means applicable community emails include the appropriate discussion URL and description.

Written by the indexing model from the issue text.