This commit changes `DiscourseIpInfo.mmdb_download` to use `File.join`
instead of `URI.join` when `GlobalSetting.maxmind_mirror_url` has been
configured. This is necessary because `URI.join` does not work the way I
expect it to work when I implemented it previously.
`URI.join("http://www.example.com/mirror", "test.tar.gz") results in
`http://www.example.com/test.tar.gz` instead of our expected
`http://www.exmaple.com/mirror/test.tar.gz`. For our simple use case
here, `File.join` is sufficient.
Why this change?
This allows downloading the MaxMind databases from a mirror in cases
where downloading directly from MaxMind's API endpoint is problematic
due to API limits.
