feat: add total of participants in activity

[MarkusDick]

Feat / Fix: #4204

Adds total of participants in activities. See below for the screenshot.

I reduced the total by one, because in the array also the contact itself is indicated, but not in the displayed list.

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[asbiin]

Great job!

[github-actions]

🎉 This PR is included in version 3.3.0 🎉

The release is available on:

• GitHub release
• v3.3.0

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