reword from "Invite Link"/"QR Code" to "Invite Code"

[r10s]

having only one term makes it easier to write and talk about it, also the UI will be less cluttered.

disadvantage is that "QR Code" implies a bit more that it is "sth to scan sometimes" - however, that hint can be given visually by a QR Code symbol beside "Invite Code", shown at least on important places (no need to make a rule out of that :)

targeting this issue requires adapting the help as well as in-app-strings