[CALENDAR] Store event attendance at flag

[chibenwa]

Description

As a user I want twake mail to remember my accendance status.

Expected result

IE

GIVEN an event with an ICS invitation
AND the user opens the mail a first time
WHEN the user accept the invitation
THEN then the 'Yes' button is marked as triggered (filled in light grey)

GIVEN an event with an ICS invitation
AND the user opens the mail a first time
AND the user accept the invitation
AND the user close the mail
WHEN the user reopens it
THEN then the 'Yes' button is marked as triggered (filled in light grey)

Idem for no + maybe

Current behavior

No state is stored this is pure shoot-and-forget.

Additional information

We shall use $Accepted, $TentativelyAccepted and $Rejected to mark the event attendance on the mail. When opening an email those keywords can be used to set the state of the bluebar.