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/main/forms.py

@@ -1,3 +1,4 @@
+import re
 import weakref
 from datetime import datetime, timedelta
 from itertools import chain
@@ -9,12 +10,17 @@ from flask_wtf import FlaskForm as Form
 from flask_wtf.file import FileAllowed
 from flask_wtf.file import FileField as FileField_wtf
 from notifications_utils.columns import Columns
-from notifications_utils.formatters import strip_whitespace
+from notifications_utils.formatters import (
+    normalise_whitespace_and_newlines,
+    remove_whitespace_before_punctuation,
+    strip_whitespace,
+)
 from notifications_utils.recipients import (
     InvalidPhoneError,
     normalise_phone_number,
     validate_phone_number,
 )
+from notifications_utils.take import Take
 from wtforms import (
     BooleanField,
     DateField,
@@ -362,6 +368,23 @@ class StripWhitespaceStringField(StringField):
         super(StringField, self).__init__(label, **kwargs)
 
 
+class StripWhitespaceTextAreaField(TextAreaField):
+    def process_formdata(self, valuelist):
+        if valuelist:
+            self.data = Take(
+                valuelist[0]
+            ).then(
+                remove_whitespace_before_punctuation
+            ).then(
+                normalise_whitespace_and_newlines
+            ).then(
+                # similar to normalise_multiple_newlines but taking everything down to one `\n` instead of two
+                lambda value: re.compile(r'\n{2,}').sub('\n', value)
+            ).then(
+                str.strip
+            )
+
+
 class OnOffField(RadioField):
 
     def __init__(self, label, choices=None, *args, **kwargs):
@@ -741,6 +764,43 @@ class SMSTemplateForm(BaseTemplateForm):
         OnlySMSCharacters()(None, field)
 
 
+class LetterAddressForm(StripWhitespaceForm):
+    MIN_ADDRESS_LINES = 3
+    MAX_ADDRESS_LINES = 7
+
+    address = StripWhitespaceTextAreaField(
+        'Address',
+        validators=[DataRequired(message="Cannot be empty")]
+    )
+
+    def validate_address(self, field):
+        lines = field.data.splitlines()
+        if len(lines) < self.MIN_ADDRESS_LINES:
+            raise ValidationError('Address must be at least 3 lines long')
+        if len(lines) > self.MAX_ADDRESS_LINES:
+            raise ValidationError('Address must be no more than 7 lines long')
+
+    @property
+    def as_address_lines_1_to_7_with_postcode(self):
+        lines = self.address.data.splitlines()
+        placeholders = {}
+
+        # set all placeholders to empty strings, or all_placeholders_in_session will always return false.
+        # note that it must be `address line #` with spaces, not underscores or dashes
+        for i in range(1, 7):
+            placeholders[f'address line {i}'] = ''
+
+        # unroll the address into lines, and place into the session in the underlying placeholder names
+        # postcode is required so make sure we put the last value in that
+        # TODO: When postcode is no longer a required field, remove this special case and just use `address line #`
+        address_lines, last_address_line = lines[:-1], lines[-1]
+        for i, line in enumerate(address_lines, start=1):
+            placeholders[f'address line {i}'] = line
+        placeholders['postcode'] = last_address_line
+
+        return placeholders
+
+
 class EmailTemplateForm(BaseTemplateForm):
     subject = TextAreaField(
         u'Subject',