notifications: send email when manual app update is required

src/mail_templates/app_manual_update_required-text.ejs

@@ -0,0 +1,15 @@
+Dear <%= cloudronName %> Admin,
+
+The following app(s) require manual update:
+
+<% for (const app of apps) { -%>
+* <%= app.title %> at <%= app.fqdn %> -> v<%= app.version %>
+<% } -%>
+
+Go to the update section of the app to update.
+
+Powered by https://cloudron.io
+
+Don't want such mails? Change your notification preferences at <%= notificationsUrl %>
+
+Sent at: <%= new Date().toUTCString() %>