registration: Make check_prereg_key return PreregistrationUser.
registration: Make check_prereg_key return PreregistrationUser.

kou/zulip,andersk/zulip,kou/zulip,kou/zulip,rht/zulip,eeshangarg/zulip,rht/zulip,andersk/zulip,andersk/zulip,zulip/zulip,andersk/zulip,andersk/zulip,kou/zulip,zulip/zulip,eeshangarg/zulip,andersk/zulip,eeshangarg/zulip,andersk/zulip,zulip/zulip,eeshangarg/zulip,rht/zulip,eeshangarg/zulip,rht/zulip,kou/zulip,rht/zulip,rht/zulip,eeshangarg/zulip,zulip/zulip,zulip/zulip,kou/zulip,rht/zulip,zulip/zulip,eeshangarg/zulip,zulip/zulip,kou/zulip