Strip obscure whitespace from form submissions

We strip most whitespace as of:
https://github.com/alphagov/notifications-admin/pull/1701

However we are still getting some bad email addresses through, for
example one that had a leading zero-width space character. This means
that the user sees a validation error; really we should just deal with
the mess for them.

So this commit also includes characters without Unicode character
property "WSpace=Y" (which includes zero-width space) to those which are
stripped from form submissions.

List taken from here: https://en.wikipedia.org/wiki/Whitespace_character

See issue and discussion here: https://bugs.python.org/issue13391

tests/app/main/test_strip_whitespace_form.py

@@ -0,0 +1,32 @@
+import pytest
+from wtforms import Form, StringField
+
+from app.main.forms import StripWhitespaceForm, StripWhitespaceStringField
+
+
+class ExampleForm(StripWhitespaceForm):
+    foo = StringField('Foo')
+
+
+class ExampleFormSpecialField(Form):
+    foo = StripWhitespaceStringField('foo')
+
+
+@pytest.mark.parametrize('submitted_data', [
+    'bar',
+    ' bar ',
+    """
+        \t    bar
+    """,
+    ' \u180E\u200B \u200C bar \u200D \u2060\uFEFF ',
+])
+@pytest.mark.parametrize('form', [
+    ExampleForm,
+    ExampleFormSpecialField,
+])
+def test_form_strips_all_whitespace(
+    app_,
+    form,
+    submitted_data,
+):
+    assert form(foo=submitted_data).foo.data == 'bar'