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)

[miaulalala]

Investigated a bit. Seems the issue is with not taking the SEQUENCE into account for recurrences.

From the iTip rfc:

1. The primary key for referencing a particular iCalendar component
   is the "UID" property value. To reference an instance of a
   recurring component, the primary key is composed of the "UID" and
   the "RECURRENCE-ID" properties.

2. The secondary key for referencing a component is the "SEQUENCE"
   property value. For components where the "UID" and
   "RECURRENCE-ID" property values are the same, the component with
   the highest numeric value for the "SEQUENCE" property obsoletes
   all other revisions of the component with lower values.

So when changing a single recurrence like in @tobiasKaminsky 's example, the highest SEQUENCE should win. It's not neccessary to check in out case though as the iMIP message will contain two VEVENTs, one with the RRULE, and one with the RECURRENCE-ID.

I need to check if this also works when I update the complete event with a recurrence exception.

[ChristophWurst]

Could we collect a list of use cases where we want to improve the email contents? So far we discussed

1. Recurring events
2. One-time event where the location (or anything similar) changed -> highlight that?
3. ???

[avinash-0007]

@ChristophWurst As right now ,Mail about "Invitation updated" doesn’t say what actually happened
In case of One time and Recurring event:
It would be useful to highlight
1)title
2)description
3)Location
4) Date

We can highlight the change using color change as well.For e.g

[miaulalala]

@ChristophWurst As right now ,Mail about "Invitation updated" doesn’t say what actually happened In case of One time and Recurring event: It would be useful to highlight 1)title 2)description 3)Location 4) Date

We can highlight the change using color change as well.For e.g

Please see nextcloud/server#35743 for the current state.

We're going to provide a diff of the changed values you mentioned above.

[miaulalala]

Closed in nextcloud/server#35743