Don't activate org-msg mode when replying to icalendar event in mu4e

[jumper047]

Mu4e also supported icalendar invitations, so I added same advice as you added into gnus function.

[johnhamelink]

I came across this PR while looking for clarity on calendar invites - great timing! Seems to work well for me (aside from the fact that when I send an email from fastmail to O365 via Davmail, gnus makes assumptions about the existence of the DTEND property in the ICS file - more info here):

• I receive a calendar invite
• The calendar invite has an [Accept] link which I activate
• A plain mu4e-compose buffer is created with <#part type="text/calendar; method=REPLY; charset=utf-8" buffer="*CAL*"><#/part> in the body.

[jumper047]

@jeremy-compostella humble ping