import: Handle signup_notifications_stream_id.
import: Handle signup_notifications_stream_id.

Previously, our realm import logic only did the special remapping
logic for the original notifications_stream_id; when we added the new
signup_notifications_stream_id field, we neglected to handle it in the
same way.

brainwane/zulip,punchagan/zulip,jackrzhang/zulip,punchagan/zulip,dhcrzf/zulip,synicalsyntax/zulip,zulip/zulip,eeshangarg/zulip,kou/zulip,timabbott/zulip,shubhamdhama/zulip,synicalsyntax/zulip,shubhamdhama/zulip,tommyip/zulip,jackrzhang/zulip,dhcrzf/zulip,dhcrzf/zulip,synicalsyntax/zulip,kou/zulip,tommyip/zulip,punchagan/zulip,timabbott/zulip,kou/zulip,hackerkid/zulip,timabbott/zulip,tommyip/zulip,eeshangarg/zulip,brainwane/zulip,jackrzhang/zulip,rishig/zulip,zulip/zulip,eeshangarg/zulip,kou/zulip,rht/zulip,showell/zulip,zulip/zulip,kou/zulip,rht/zulip,zulip/zulip,punchagan/zulip,tommyip/zulip,brainwane/zulip,punchagan/zulip,brainwane/zulip,showell/zulip,eeshangarg/zulip,shubhamdhama/zulip,andersk/zulip,dhcrzf/zulip,zulip/zulip,punchagan/zulip,shubhamdhama/zulip,timabbott/zulip,eeshangarg/zulip,rishig/zulip,andersk/zulip,andersk/zulip,showell/zulip,hackerkid/zulip,jackrzhang/zulip,tommyip/zulip,rht/zulip,hackerkid/zulip,tommyip/zulip,andersk/zulip,kou/zulip,timabbott/zulip,brainwane/zulip,dhcrzf/zulip,punchagan/zulip,rht/zulip,rht/zulip,andersk/zulip,shubhamdhama/zulip,zulip/zulip,kou/zulip,hackerkid/zulip,showell/zulip,hackerkid/zulip,shubhamdhama/zulip,jackrzhang/zulip,brainwane/zulip,rht/zulip,jackrzhang/zulip,rishig/zulip,showell/zulip,andersk/zulip,synicalsyntax/zulip,timabbott/zulip,synicalsyntax/zulip,synicalsyntax/zulip,dhcrzf/zulip,eeshangarg/zulip,brainwane/zulip,rishig/zulip,hackerkid/zulip,jackrzhang/zulip,rht/zulip,rishig/zulip,tommyip/zulip,synicalsyntax/zulip,timabbott/zulip,hackerkid/zulip,eeshangarg/zulip,dhcrzf/zulip,showell/zulip,zulip/zulip,showell/zulip,rishig/zulip,andersk/zulip,shubhamdhama/zulip,rishig/zulip