Add form field for a UK mobile phone number

This field does two things:
- validates the format of the phone number
- outputs a consistent representation of the phone number

Because of this I think it’s better represented as a new field type, rather
than individual validators.

I also think that it’s better to do this without regular expression(s), because
it makes returning the specific error easier.

This commit also adds basic pass/fail test for a series of valid/invalid
phone numbers.

tests/app/main/test_phone_number_form_field.py

@@ -0,0 +1,68 @@
+import pytest
+from wtforms import Form
+from app.main.forms import UKMobileNumber
+
+
+class FormExample(Form):
+    phone_number = UKMobileNumber()
+
+phone_numbers = {
+    'invalid': [
+        # Too long
+        '0712345678910',
+        '0044712345678910',
+        '0044712345678910',
+        '+44 (0)7123 456 789 10',
+        # Too short
+        '0712345678',
+        '004471234567',
+        '00447123456',
+        '+44 (0)7123 456 78',
+        # Not mobile (from https://fakenumber.org/generator/freephone)
+        '08081 570364',
+        '+44 8081 570364',
+        '0117 496 0860',
+        '+44 117 496 0860',
+        '020 7946 0991',
+        '+44 20 7946 0991',
+        # Contains non-numbers
+        '07890x32109',
+        '07123 456789...',
+        '07123 ☟☜⬇⬆☞☝',
+        '07123☟☜⬇⬆☞☝',
+        '07";DROP TABLE;"',
+        '+44 07ab cde fgh',
+    ],
+    'valid': [
+        '07123456789',
+        '07123 456789',
+        '07123-456-789',
+        '00447123456789',
+        '00 44 7123456789',
+        '+447123456789',
+        '+44 7123 456 789',
+        '+44 (0)7123 456 789'
+    ]
+}
+
+
+@pytest.mark.parametrize("phone_number", phone_numbers['valid'])
+def test_phone_number_accepts_valid_values(phone_number):
+    form = FormExample(phone_number=phone_number)
+    form.validate()
+    assert form.errors == {}
+
+
+@pytest.mark.parametrize("phone_number", phone_numbers['invalid'])
+def test_phone_number_rejects_invalid_values(phone_number):
+    form = FormExample(phone_number=phone_number)
+    form.validate()
+    print(phone_number)
+    assert form.errors != {}
+
+
+@pytest.mark.parametrize("phone_number", phone_numbers['valid'])
+def test_phone_number_outputs_in_correct_format(phone_number):
+    form = FormExample(phone_number=phone_number)
+    form.validate()
+    assert form.phone_number.data == '+44 7123 456 789'