Link to license upgrade screen from upgrade error page doesn't take context path into account

Steps to reproduce.

1. Upgrade from 3.0.2->3.1 on a server running on a context path. For example, http://localhost:8080/confluence
2. When navigating to Confluence in your browser, you should get a message saying that you need to upgrade your Confluence license.
3. Click on the link to enter your new license.

This will take you to a 404 page because the context path is missing. Add the correct context path and it works.