create branch 2.8

--HG--
branch : 2.8
Convert Attendee into Mixin

Since the removal of _inherits, calendar.attendee doesn't exist, so Attendee
must be converted into Mixin and used for calendar.event.attendee.

issue3662
review3601002
(grafted from 8d9b1bb00b488852b1d0b4b11b9359df509a26a6)

--HG--
branch : 2.8
extra : source : 8d9b1bb00b488852b1d0b4b11b9359df509a26a6