Add option to hide "Last Online"-timestamp / indicator in forum
See https://forum.nim-lang.org/t/6810

> Having an option to hide your "Last Online" status in the forum would be nice for privacy reasons. 

1. user profiles display when the last visit of a users was (e.g. 2h ago)
2. on the homepage participants of a thread are shown. Users that are currently only are marked with a little dot in the bottom right corner of their profile image.

~Is likely~ implemented as `deltaT = currentTime - timeOfLastUserRequest, deltaT  < threshold` (see https://github.com/nim-lang/nimforum/blob/master/src/frontend/user.nim#L28-L29)

Users should have an option in the settings to disable this behavior. A simple implementation could simply hide the information mentioned in 1) and 2), the preferred solution should avoid the collection of a  "`timeOfLastUserRequest`"-timestamp in the first place.
