input letter address data in a single block

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.

app/navigation.py

@@ -259,6 +259,7 @@ class HeaderNavigation(Navigation):
         'send_messages',
         'send_notification',
         'send_one_off',
+        'send_one_off_letter_address',
         'send_one_off_step',
         'send_test',
         'no_cookie.send_test_preview',
@@ -386,6 +387,7 @@ class MainNavigation(Navigation):
             'manage_template_folder',
             'send_messages',
             'send_one_off',
+            'send_one_off_letter_address',
             'send_one_off_step',
             'send_test',
             'no_cookie.send_test_preview',
@@ -673,6 +675,7 @@ class CaseworkNavigation(Navigation):
             'choose_from_contact_list',
             'choose_template',
             'send_one_off',
+            'send_one_off_letter_address',
             'send_one_off_step',
             'send_test',
             'send_test_step',
@@ -1156,6 +1159,7 @@ class OrgNavigation(Navigation):
         'send_messages',
         'send_notification',
         'send_one_off',
+        'send_one_off_letter_address',
         'send_one_off_step',
         'send_test',
         'no_cookie.send_test_preview',