Bug 933192 - [Settings] can join hidden wifi network without ssid name r=ejchen

Bug 933192 - [Settings] can join hidden wifi network without ssid name r=ejchen
nit: "disable the 'Join' button when any of the fields are not valid"

You can explain how the fields are not valid as a comment on the function when externalized (comment below - https://github.com/mozilla-b2g/gaia/pull/20284/files#discussion_r13590454)