Mailing list unavailability message shouldn't be an alert

[Matthew Paul Thomas]

1. Log in to launchpad.dev as <email address hidden>.
2. Go to <http://launchpad.dev/~landscape-developers/+contactaddress>.

The text "Not available because you haven't applied for a mailing list" is presented as an informational alert. As specified in <https://launchpad.canonical.com/AlertMessages>, this is wrong, because nothing unusual has happened -- in fact nothing at all has happened.

Instead, the text should be a caption for the radio button label -- smaller text, with no special style. (The label itself should also be greyed out.)