luci-mod-network: use createHanleFn for handleJoin function

handleJoin can be slow to parse all the data and show the new modal, this can result in the disappear of the scan modal and the showing of the wifi config page while the data are generating. This is wrong since a user can think that he did something wrong. Fix this by using the createHandleFn and by removing the scan pool function instead of calling ScanAbort function that with the other thing wrongly removes the Scan modal. (the modal is replaced with the add one when all the data are ready)

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>