Make display of incoming PM configurable

[cblgh]

Currently PMs will show up as an incoming PM (like being whispered in games like World of Warcraft) but some people find this disconcerting.

Let's put that display behaviour behind a configuration (like we've done with the timestamps).

[hackergrrl]

Open Q: what ought the default be?

[cblgh]

my take: opt-out (on by default), because otherwise discoverability of the feature is difficult. there are arguments for opt-in too, ofc, but mainly seeking to share my opinion and not argue my case :)

[hackergrrl]

My main concern is not wanting to cause confusion around PMs, especially for news users. If people see PM contents in public channels, it could undermine their trust in the app. So my preference would be off by default (opt-in).