Topic/migrator user management

How about this:
```
Hi {{if possible {{First Name}}, or otherwise {{old otm1 site name}} user}},

Thanks for using {{old otm1 site name}}! We recently upgraded to the newest version
of the OpenTreeMap software that powers the site ("OTM2"). We noticed there were multiple
accounts created in the old site for your email, "welrood@hotmail.com". This is not permitted in OTM2,
so we have merged the activity of your multiple accounts onto one, "welrood".
If you have any difficulty signing in, please contact us at {{site contact email}}.
```

I assume this is a genericized user migrator for more than just PhillyTreeMap?

I would want to keep references to OTM1 and OTM2 at a minumum, because end users frequently don't know what that means. Use the old/new site names.