FIX: allows PM owner to remove any user if >= TL2

[jjaffeux]

• < TL2 user can remove itself:

• >= TL2 user can remove everyone:

[ZogStriP]

What happens if OP remove themselves from the PM? 🤔

[jjaffeux]

What happens if OP remove themselves from the PM? 🤔

replaced by system user

So we keep trace of the OP being crated by this user, but we replace its presence in the topic by system. Also "bob" here, can no longer access/see this.

Note 1: the screenshot is done with a staff account, so this is why you can see "Add or Remove"
Note 2: the small action text is a little bit weird: "Bob removed themselves....", I think it's ok though, very rare use case...