Fix parsing of attendee when common name is missing

[sanketfirodiya]

Parsing would be incorrect if the attendee had CN missing, e.g.:

ATTENDEE;CUTYPE=RESOURCE;ROLE=REQ-PARTICIPANT;PARTSTAT=ACCEPTED;X-NUM-GUEST
 S=0:mailto:superhuman.com_3633323632323332383537@resource.calendar.google.c
 om

vs when its not missing:

ATTENDEE;CUTYPE=INDIVIDUAL;ROLE=REQ-PARTICIPANT;PARTSTAT=NEEDS-ACTION;RSVP=
 TRUE;CN=rahul@superhuman.com;X-NUM-GUESTS=0:mailto:rahul@superhuman.com

This was happening because we reuse the placeholder string to parse out different components. I think its cleaner and less error prone if we parse out each component in its own string.

Fixes https://github.com/superhuman/superhuman-ios/issues/2374