Support schema.org one-click and go-to actions on e-mails

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See: https://gitlab.gnome.org/GNOME/evolution/-/issues/1510 for a summary, and https://developers.google.com/gmail/markup/reference/go-to-action#json-ld for a description of example actions

Emitting appropriate metadata (and implementing various handlers) could enable deep integration with email clients, allowing better experiences for users.

We should support links for all notifications that implement the view it in GitLab as a go-to-action.

Some candidates for http-handler actions would include approve MR, close issue, resolve discussion.