Add more functions to notification template parsing

[justinbeaty]

I wanted to add a copyright notification in the footer of my newsletters. I can use the Date function in the campaign body, but not in any of the static/email-templates/*.html files.

 {{ define "footer" }}
     </div>
     
     <div class="footer">
         <p>{{ L.T "public.poweredBy" }} <a href="https://listmonk.app" target="_blank">listmonk</a></p>
+        <small>&copy; {{ Date "2006" }}</small>
     </div>
     <div class="gutter">&nbsp;</div>
 </body>
 </html>
 {{ end }}

Try to add the above modification, and:

$ ./listmonk --static-dir=static
2023/03/25 15:30:13 main.go:99: v2.4.0 (c668523 2023-03-20T13:50:31Z)
2023/03/25 15:30:13 init.go:138: reading config: config.toml
2023/03/25 15:30:13 init.go:266: connecting to db: /var/run/postgresql/:5432/listmonk
2023/03/25 15:30:14 init.go:225: loading static files from: static
2023/03/25 15:30:14 init.go:585: media upload provider: filesystem
2023/03/25 15:30:14 init.go:615: error parsing e-mail notif templates: template: base.html:94: function "Date" not defined

This PR adds in the Date function, as well as sprig functions.