feat: Ability to render groups as attendees

Implement ability to render groups and group members as attendees of an
calendar event. This is implemented by reading and interpreting the
CUTYPE and MEMBER parameter of the iCal ATTENDEE attribute.

For example attendee "user1" belongs to a group identified with the uri
mygroup@localhost: "ATTENDEE;MEMBER="mailto:mygroup@localhost";CN=user1;[...]".
The group entity is also a ATTENDEE entry but with "CUTYPE=GROUP" and
with the corresponding uri mygroup@localhost:
"ATTENDEE;CN=MyGroup;CUTYPE=GROUP;[...]:mailto:mygroup@localhost".

In the frontend, groups will be displayed as a collapsible list whereas
members of the group will be displayed with the same functionality as
normal attendees.

In case the group entry gets deleted, all of its members will get
removed too.

Empty groups with no members might be present in the iCal file but won't
get rendered.

According to the iCal specifications, groups can have more than one uri and
attendees could have multiple membership addresses in one attribute.
This is neither covered yet in calendar-js nor in this PR.

Signed-off-by: Jonas Heinrich <heinrich@synyx.de>