notify: Emails formatted by templates

At present, email bodies are simple strings constructed by the sender, which limits their composition.

It is proposed to have a `WithTemplates` option, whereby templates are optionally supplied to the Notifier.

The notifier would include an optional property:

`templates *template.Template`

Templates would be keyed by the notification **kind**.

A new `FmtSend`, that wraps `Send` method, would be implemented roughly as follows:

```
func (n *Notifier) FmtSend(ctx context.Context, skey int64, kind string, data interface{}) error {
if n.templates == nil {
return errors.New("missing templates")
}
var b bytes.Buffer
// Look up the template for this kind of notification.
err = n.templates.ExecuteTemplate(&b, kind, data) {
if err != nil {
// Handle error
return err
}
return n.Send(ctx, skey, kind, b.String())
}

```