Add "Tentative" button to calendar invitation emails

[!TIP]

Help move this idea forward

• Use the 👍 reaction to show support for this feature.
• Avoid commenting unless you have relevant information to add; unnecessary comments create noise for subscribers.
• Subscribe to receive notifications about status changes and new comments.

Feature Request

Currently, calendar invitation emails sent by Nextcloud only include "Accept" and "Decline" buttons. To improve flexibility for attendees, a "Tentative" button should be added, which sets the PARTSTAT to TENTATIVE in the iCalendar object (RFC 5545).

Why?

• Many users need a "maybe" option before confirming attendance.
• TENTATIVE is a standard value in the CalDAV/iCalendar specification.
• Other calendar services (e.g., Google Calendar, Outlook) support this.

Proposed Changes

1. Add a "Tentative" button to the email template (apps/dav/templates/email/invitation.html).
2. Generate a tentativeUrl in the email notification logic (apps/dav/lib/Notification/EmailNotification.php).
3. Add a new route and controller method (InvitationController::tentative()) to handle the tentative response.
4. Update the attendee's PARTSTAT to TENTATIVE in the iCalendar object.

Technical Details

• The change affects the DAV app (CalDAV backend) in the Nextcloud server.
• The PARTSTAT property in the ATTENDEE field of the iCalendar object should be set to TENTATIVE.
• The email template and notification logic need to be extended.

Related Issues

• #2760 (Send email invitation to organiser)
• #340 (Send event description in invitation emails)

Start with apps/dav/templates/email/invitation.html and apps/dav/lib/Notification/EmailNotification.php, then trace the invitation route and InvitationController response methods. Add the tentative action so the email offers the new response and the attendee's iCalendar PARTSTAT becomes TENTATIVE; done means the invitation flow supports that response alongside Accept and Decline.