AddressPickerDialog mark addresses already in the room/group

[t3chguy]

grey them out and deprioritize them
to make it easier to see if you're trying to invite someone/thing already in a room/group

Second half of abandoned matrix-org/matrix-react-sdk#1678

[lampholder]

I thought there was already an issue for this, but perhaps I was thinking of #4473

[t3chguy]

or #5792 which is similar

[t3chguy]

How about something like this:

Takes up quite a bit of space, maybe just a horizontal rule through it with a tooltip on hover?