Let users register with int’national phone numbers

Right now Notify restricts you to registering with a UK mobile number.
This is because when we built the user registration stuff we couldn’t
send to international mobiles.

However we can send to international mobile numbers, and it’s totally
reasonable to expect employees of the UK government to be working
abroad, and have a foreign mobile phone – we’ve heard from one such
user.

So this commit:
- changes all places where users enter their own phone number to use
  the validation function which allows international phone numbers
- renames the `mobile_number` validation to `uk_mobile_number` to make
  it explicit, and force it to break the tests if there’s somewhere it’s
  being used that I haven’t thought of

app/main/forms.py

@@ -114,7 +114,7 @@ class InternationalPhoneNumber(TelField):
             raise ValidationError(str(e))
 
 
-def mobile_number(label='Mobile number'):
+def uk_mobile_number(label='Mobile number'):
     return UKMobileNumber(label,
                           validators=[DataRequired(message='Can’t be empty')])
 
@@ -156,14 +156,14 @@ class RegisterUserForm(Form):
     name = StringField('Full name',
                        validators=[DataRequired(message='Can’t be empty')])
     email_address = email_address()
-    mobile_number = mobile_number()
+    mobile_number = international_phone_number()
     password = password()
 
 
 class RegisterUserFromInviteForm(Form):
     name = StringField('Full name',
                        validators=[DataRequired(message='Can’t be empty')])
-    mobile_number = mobile_number()
+    mobile_number = international_phone_number()
     password = password()
     service = HiddenField('service')
     email_address = HiddenField('email_address')
@@ -210,7 +210,7 @@ class EmailNotReceivedForm(Form):
 
 
 class TextNotReceivedForm(Form):
-    mobile_number = mobile_number()
+    mobile_number = international_phone_number()
 
 
 class ServiceNameForm(Form):
@@ -329,7 +329,7 @@ class ChangeEmailForm(Form):
 
 
 class ChangeMobileNumberForm(Form):
-    mobile_number = mobile_number()
+    mobile_number = international_phone_number()
 
 
 class ConfirmMobileNumberForm(Form):
@@ -587,7 +587,7 @@ class Whitelist(Form):
     )
 
     phone_numbers = FieldList(
-        UKMobileNumber(
+        InternationalPhoneNumber(
             '',
             validators=[
                 Optional()
@@ -669,7 +669,7 @@ def get_placeholder_form_instance(
         if allow_international_phone_numbers:
             field = international_phone_number(label=placeholder_name)
         else:
-            field = mobile_number(label=placeholder_name)
+            field = uk_mobile_number(label=placeholder_name)
     elif optional_placeholder:
         field = StringField(placeholder_name)
     else: