Fix wrong formats of ACTIVITY-Listing in session

Closes #1875 (closed), #1876 (closed)

What does this MR do?

The MR showed that invalid data is stored in sessions or DB. The ACTIVITIES-LIST in session contains invalid JSON.

Additional was a problem with JSON deserialisation of a complexer structure of the array. Default behaviour creates an stdClass which can not be iterated. This is fixed by using json_decode with flag to convert data into assoziative arrays.

How confident are you it won't break things if deployed?

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• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
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• added an entry to CHANGELOG.md
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