Add log field and helpers for StatisticsService

[Greenscreen23]

• A log entry contains time, tags for everything that might be relevant to the event, and some description of the event. This description should be configurable. One cheap option is to start with either plain text descriptions or a map of plain text description fragments stiched together in the frontend.
• Tags are objects containing a display name, a category (patient, patient category, simulatedRegion, ...) and a specifier custom to this category. All these fields are strings. Maybe also a color. Maybe not. The specific tags are not part of this issue.
• The state contains an optional and unvalidated field for an array of log entries.
• There are helpers for use in reducers that depend on the state and add log entries.
• If the array is there, the helpers add entries, otherwise they do nothing.
• In particular, the helpers must not calculate and discard the log entry but only compute it if required.