Use [create_account_url] from strings.xml consistently in notifications.xml.

The URL whose simple form is http://join.secondlife.com actually appears in a
number of places in the localized notifications.xml files. It's supposed to be
localized for the current viewer language -- and in strings.xml, it is. But
the same URL is restated a couple times in notifications.xml, sometimes
localized, sometimes not.
Add "create_account_url" to init_default_trans_args(), permitting us to embed
[create_account_url] anywhere in the skin files. Then replace all known
occurrences of that URL in notifications.xml files with [create_account_url].