Only calculate the room name once instead of loading all participants

[nickvergessen]

Steps

1. Create 70 users:

i=10
while [ $i -ne 70 ]
do
        i=$(($i+1))
        echo "$i"

	USERID=$(uuidgen)
	OC_PASS="123456" occ -vvv user:add --password-from-env --display-name "Dummy #$i" -g "dummies" $USERID
	occ -vvv user:setting $USERID settings email "test$i@yourdomain.tld"

done

2. Create a conversation as admin with group "dummies" (and check the database queries of this request)

Left is before, right is with this patch

[danxuliu]

Code looks good based on the use of userId in resolveRoomDisplayName() (although I have not tested it).