When a site has `SiteSetting.invite_only` enabled, we create a
`ReviewableUser`record when activating a user if the user is not
approved. Therefore, we need to approve the user when redeeming an
invite.
There are some uncertainties surrounding why a `ReviewableRecord` is
created for a user in an invites only site but this commit does not seek
to address that.
Follow-up to 7c4e2d33fa4b922354c177ffc880a2f2701a91f9