saml: Extract logic for determining if subdomain is allowed by an attr.
saml: Extract logic for determining if subdomain is allowed by an attr.

This logic has some trickiness due to the need to handle root subdomain
aliases, so this is worth extracting into a helper for use in other
backends.

andersk/zulip,rht/zulip,showell/zulip,kou/zulip,eeshangarg/zulip,hackerkid/zulip,andersk/zulip,zulip/zulip,andersk/zulip,showell/zulip,showell/zulip,zulip/zulip,hackerkid/zulip,punchagan/zulip,eeshangarg/zulip,punchagan/zulip,zulip/zulip,hackerkid/zulip,andersk/zulip,rht/zulip,punchagan/zulip,andersk/zulip,eeshangarg/zulip,punchagan/zulip,eeshangarg/zulip,eeshangarg/zulip,showell/zulip,showell/zulip,kou/zulip,rht/zulip,kou/zulip,showell/zulip,andersk/zulip,zulip/zulip,kou/zulip,hackerkid/zulip,hackerkid/zulip,hackerkid/zulip,eeshangarg/zulip,kou/zulip,kou/zulip,rht/zulip,punchagan/zulip,punchagan/zulip,rht/zulip,zulip/zulip,hackerkid/zulip,eeshangarg/zulip,rht/zulip,punchagan/zulip,zulip/zulip,kou/zulip,showell/zulip,rht/zulip,zulip/zulip,andersk/zulip