Adjust update email text to let the user know about auto updates

So far the emails make no sense in case the user only clicks the update
link after the Cloudron has already auto updated. We might want to
adjust the text, once the user can disable auto updates, or set the
preferred update time.

src/mail_templates/box_update_available.ejs

@@ -4,7 +4,7 @@ Dear Admin,
 
 A new version of Cloudron <%= fqdn %> is available!
 
-Please update at your convenience at <%= webadminUrl %>.
+Your Cloudron will perform an automatic update over night, or manually update at your convenience at <%= webadminUrl %>.
 
 Changelog:
 <% for (var i = 0; i < changelog.length; i++) { %>
@@ -12,7 +12,7 @@ Changelog:
 <% } %>
 
 Thank you,
-Update Manager
+your Cloudron
 
 <% } else { %>