rather than in multiple placeholders - this is the first step towards
making postcodes non-required, which is the first step towards
international letters.
they still populate address_line_# and postcode fields under the hood -
to keep validation working the same, the last line always goes into
`postcode`.
the form normalises whitespace, removes extra new lines, and enforces
that you have between three and seven lines.
if the letter repeats address placeholders further down (eg "Dear
((address_line_1))"), then it'll fill those in as well. It'll still
prompt you to fill them in, but they'll be pre-filled.
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