Added Vietnamese address format

Accounts for both rural and urban address formats.
Corrected Vietnamese address preset

Nothing in Vietnam uses addr:suburb, and it seems to be intended to be subordinate to addr:district and addr:subdistrict, which means it definitely doesn’t map to thị xã. I also found some examples using addr:district for thị xã. So addr:district should be used for both quận and thị xã in a municipality.

The resulting placeholder text will be rather long, and Vietnamese mail tend to have each component on a separate line, so I also split subdistrict, district, and city onto separate lines.
Merge pull request #2252 from 1ec5/patch-1

Vietnamese address preset