Improve invitation update notification email

[st3iny]

Is your feature request related to a problem? Please describe.
When responding to an invitation via an external client or my new feature, our CalDAV backend will send a notification to the organizer. However, this notification is very crude and could be improved.

Describe the solution you'd like
Show more information about what actually changed about the event. We could start by adding information about the invitee that reacted.

Describe alternatives you've considered
None

Additional context
How the notification email currently looks:

The term "Einladung aktualisiert" means "invitation was updated" in English.

[ChristophWurst]

https://github.com/orgs/nextcloud/projects/45#card-74890297 guess this materializes that card into a ticket. Thanks :)

[tobiasKaminsky]

Same happens when:

• there is a monthly appointment
• one appointment is moved from e.g. Wednesday to Friday
• email is:
• which shows start of appointment rule, but not what appointment (today's) was changed to what (moved to Friday, different time)