Invite is used in two contexts, when inviting a new user to the forum
and when inviting an existent user to a topic. The first case is more
complex and it involves permission checks to ensure that new users can
be created. In the second case, it is enough to ensure that the topic
is visible for both users and that all preconditions are met.
One edge case is the invite to topic via email functionality which
checks for both conditions because first the user must be invited to
create an account first and then to the topic.
A side effect of these changes is that all site settings related to
invites refer to inviting new users only now.
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