Missing charset in iCalendar events attached to reminders

[jhyeon]

Describe the bug
Indico 3.1 does not specify the charset parameter in an iCalendar event attached to reminder email. More precisely, it seems to misname the parameter encoding.

To Reproduce
Steps to reproduce the behavior:

1. Create an event
2. Add a reminder
3. Check the "Attach iCalendar file" option
4. Receive the reminder email
5. Some applications fail to handle the iCalendar event in the message

Expected behavior
The iCalendar event is expected to be displayed by the email application.

Additional context
Section 3.1.4 of RFC5545 requires the charset Content-Type parameter in MIME transports. See also the errata. This rule was obeyed by previous releases of indico as the following MIME part shows:

--===============9109531323641687849==
Content-Type: text/calendar; charset="utf-8"
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Content-Disposition: attachment; filename="event.ics"

BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//CERN//INDICO//EN

This is no longer the case as of version 3.1:

--===============5843370041342135681==
Content-Type: text/calendar; filename="event.ics"; encoding="utf-8";
 method="REQUEST"
MIME-Version: 1.0

BEGIN:VCALENDAR
VERSION:2.0
PRODID:-//CERN//INDICO//EN

Another parameter called encoding is set to utf-8 instead.