Follow specific person writing during mapping

**User input 1 (by Karen-Maria Andersson - sharepoint ID185):**

During mapping I can see that NN updated a card or a PQIR. Would like to be able to follow one specific user. Useful when co-facilitating mapping

Start by tracing how mapping updates to cards and PQIRs are displayed and how the updating user's identity is exposed. Define the interaction for selecting one person to follow, then verify that updates from that person are surfaced during mapping without including unrelated users.

Written by the indexing model from the issue text.