User search filter - View online users only

Is your feature request related to a problem? Please describe.

There seems to be a need to have a fast accessible overview of all users that are currently online on an instance. The user search can be accessed fast, but it takes to long to lookup for an user just to see if he or she is online.
e.g. Bob wants to ask Alice if she wants to join him on lunch. Bob needs to open the search first, starts typing ‘Ali’, waits for the DB to return the list and then he can send Alice a direct message. But if Alice is offline and he would have seen that before he would have saved the time he wasted on the user search.

Describe the solution you’d like
As described in #7604 a filter for the directory or user search which would show only the online users would be a good way to go providing that this filter could be accessed with a shortcut.

Describe alternatives you’ve considered
An alternative would be a scroll able user online list in the sidebar (maybe beneath the conversations?)