When custom domain is used, use correct domain to set cookies

When using a custom domain that is a subdomain (e.g. feedback.example.com), some cookies are set on the parent domain (example.com).

This is an issue for various reasons (including security related).

In our case, since the cookies the featurebase app sets are quite big (>1600 bytes in our case), these cookies are inadvertently also sent to all our sites that are hosted on another subdomain of example.com [http://example.com]. Because of it, these other services suffer from issues such as “431 request entity too large”. Because the total allowed header size (typically 4k) has to fit all headers, including the cookies these services may need themselves.

Can this be fixed?