1840: Properly serialize activity log and fix it

[steffenkleinle]

Short description

The activity log was broken for a while due to serializing not happening properly. This introduces proper typing and serializing.

Proposed changes

• Add fromSerialized and serialize methods to each extension and for cards
• Properly serialize activity log and use iso8601

Side effects

• Old activity logs break the serializing (but this should not happen as they are only session based)

Testing

See issue.

Resolved issues

Fixes: #1840

[steffenkleinle]

The additional fromSerialized and serialize methods for extensions are only necessary if user display strings are different form serialized values. Currently, this is only the case for dates (dd.MM.yyyy vs yyyy-MM-dd (ISO8601)). Obviously, we could also serialize dates as dd.MM.yyyy but I think in the long term this is better and provides more flexibility. Let me know if you disagree.