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/new_device_logged_in.hbs

@@ -9,6 +9,6 @@ New Device Logged In From {{device}}
    Device Type: {{device}}
 
    You can deauthorize all devices that have access to your account from the
-    <a href="{{url}}">web vault</a> under Settings > My Account > Deauthorize Sessions.
+    <a href="{{url}}/">web vault</a> under Settings > My Account > Deauthorize Sessions.
 </p>
 </html>