Reader-event processing and tag positioning are parameterized by TagInfo

This allows us to keep sending real-time positions to Di Colore servers during replays.
Remove string-based mostReventEventTime from TagsInfo

Client can compute it from TagsInfo.mostRecentEventTimeMs
Refactoring: don't send entire ServerState, but TagsServerInfo

So we can have properties in the state that are not sent to the client.
Introduce TagsState that may contain more properties than TagsInfo
Refactor: rename tagsInfo references to tagsState
Remove superfluous property from initialTagsServerInfo
Adapt to new ServerState
Show event date if not today