add expiry to welcome email, add dummy emailer for debugging

the "yourAccountWillExpire" has also been added to the editor for #81.
To use the dummy emailer, set [email]/method to "dummy".

mail/welcome.mjml

@@ -64,6 +64,7 @@
             <p>{{ .youCanLoginWith }}:</p>
             {{ .jellyfinURLString }}: <a href="{{ .jellyfinURLVal }}">{{ .jellyfinURL }}</a>
             <p>{{ .usernameString }}: <i>{{ .username }}</i></p>
+          	<p>{{ .yourAccountWillExpire }}</p>
         </mj-text>
       </mj-column>
     </mj-section>

mail/welcome.txt

@@ -6,5 +6,6 @@
 
 {{ .usernameString }}: {{ .username }}
 
+{{ .yourAccountWillExpire }}
 
 {{ .message }}