Update emails

Scroll at the bottom of https://lanternctrltest.appspot.com/admin/index.jsp to see it in action.

This allows you to send an update e-mail either to a single user (for testing purposes) or to all LanternUsers.  You need to provide a version string, which shows up in the notification emails (interpolated as *|VERSION|*), and is not used for anything else.

IIUC transifex will pull the strings and submit them for translation as soon as this gets merged into master.  So if you want changes in the template (as it stands it's just a blatant copy of the invite email template with just the headers changed) you better ask (or just do them yourself) before this gets merged, so as not to burden our translators unnecessarily.

For the time being, I put English placeholder text in the other languages, since the script to generate emails expects *some* text there.