Update L$ Buy to link to accounts billing instead of lindex

[akleshchev]

Description

Update L$ Buy to link to accounts billing instead of lindex.
Update notification to match new link

Old link and AddPaymentMethod notification:
https://secondlife.com/my/lindex/buy.php

New link:
https://accounts.secondlife.com/billing_information
There is no 'place order' there, so notification needs to be updated.

Scenario:
have no billing information on file, click 'buy L$', click 'get started'.

[Dan-Linden]

Need the new message text for the dialog.

For the benefit of 3rd party viewers, should payment team redirect from the old link to the new one?