posterhall: annotate (select?) posters I have already/not visited

[yarikoptic]

might be prohibitively complex to implement at this point, but thought to check on feasibility since may be DB keeps track already of which posters a user has visited. If yes -- poster previews could get some annotation (checkmark icon or alike; blueish border is taken already for speaker attendance). This would be super useful in particular to mitigate current UX shortcomings of being unable to open a poster in a new tab + unable to come back to the prior state of the posterhall (#1477): if I perform search, get e.g. 10 posters, go to one, come back, re-search, go through the list to remember which I already attended, and go to the next one. At least with the clear annotation, it would make the last step easier.

if such information is available in DB, it might be even worthwhile adding a filter with choices: "All" (default), "visited", "not visited", and ideally memoizing that selection as well (#1477), which could help to navigate through the posters.

edit1: oh hoh -- there is "users[].enteredVenueIds"! so information is there!