Add Gmail Actions to subscription emails

#### Is your feature request related to a problem? Please describe.

I receive individual notifications for posts on blogs I am subscribed if they use the Jetpack subscription feature. For example, make.wordpress.org/core and wordpress.org.

Rather than reading these in the email client, I tend to click through to the original source. At the moment this requires opening the email and clicking the link on the heading.

#### Describe the solution you'd like

To remove a step to get to the original site in Gmail, I'd like to suggest that the Go-To action markup be added to the HTML. These display the links in the subject line, similar to those seen on GitHub issues and pull requests.

The documentation is at https://developers.google.com/gmail/markup/reference/go-to-action, it looks like a few attributes would need to be added to the existing markup around the post's title.

#### Describe alternatives you've considered

#### Additional context


No file or test is named in the issue. Locate the subscription email HTML and the existing markup around the post title, then review Gmail's Go-To action requirements. Done means the generated subscription emails include valid Go-To action markup and the existing email checks still pass.

Written by the indexing model from the issue text.