Add backend support for alternate base dir (subdir/subpath) hosting

To use this, include a path in the `DOMAIN` URL, e.g.:

* `DOMAIN=https://example.com/custom-path`
* `DOMAIN=https://example.com/multiple/levels/are/ok`

src/static/templates/email/welcome_must_verify.hbs

@@ -2,7 +2,7 @@ Welcome
 <!---------------->
 <html>
 <p>
-Thank you for creating an account at <a href="{{url}}">{{url}}</a>. Before you can login with your new account, you must verify this email address by clicking the link below.
+Thank you for creating an account at <a href="{{url}}/">{{url}}</a>. Before you can login with your new account, you must verify this email address by clicking the link below.
 <br>
 <br>
 <a href="{{url}}/#/verify-email/?userId={{user_id}}&token={{token}}">