Be strict about similar email addresses for alerts

We don’t want a single person to have two accounts on an emergency
alerts service because it would let them circumvent the two eyes
approval process.

We can go some way to mitigating against this by stopping people using
common methods that email providers use to alias email addresses. These
are:
- being case insensitive
- being insensitive to the position or number of dots in the local part
  of an email address
- using ‘plus addressing’

We already prevent the first one, this commit adds normalisation which
strip out the second two before doing the comparision with the current
user’s email address.

tests/app/main/views/test_manage_users.py

@@ -1443,6 +1443,70 @@ def test_user_cant_invite_themselves(
     assert not mock_create_invite.called
 
 
+@pytest.mark.parametrize('email_address', (
+    'test@user.gov.uk',
+    'TEST@user.gov.uk',
+    'test@USER.gov.uk',
+    'test+test@user.gov.uk',
+    'te.st@user.gov.uk',
+    pytest.param('test2@user.gov.uk', marks=pytest.mark.xfail),
+    pytest.param('test@other.gov.uk', marks=pytest.mark.xfail),
+))
+def test_broadcast_user_cant_invite_themselves_or_their_aliases(
+    client_request,
+    service_one,
+    mocker,
+    active_user_with_permissions,
+    mock_create_invite,
+    mock_get_template_folders,
+    email_address,
+):
+    service_one['permissions'] += ['broadcast']
+    page = client_request.post(
+        'main.invite_user',
+        service_id=SERVICE_ONE_ID,
+        _data={
+            'email_address': email_address,
+            'permissions_field': []
+        },
+        _expected_status=200,
+    )
+    assert normalize_spaces(page.select_one('span.govuk-error-message').text) == (
+        "Error: You cannot send an invitation to yourself"
+    )
+    assert mock_create_invite.called is False
+
+
+@pytest.mark.parametrize('extra_service_permissions', (
+    pytest.param([], marks=pytest.mark.xfail),
+    ['broadcast'],
+))
+def test_platform_admin_cant_invite_themselves_to_broadcast_services(
+    client_request,
+    service_one,
+    mocker,
+    platform_admin_user,
+    mock_create_invite,
+    mock_get_template_folders,
+    extra_service_permissions,
+):
+    service_one['permissions'] += extra_service_permissions
+    client_request.login(platform_admin_user)
+    page = client_request.post(
+        'main.invite_user',
+        service_id=SERVICE_ONE_ID,
+        _data={
+            'email_address': platform_admin_user['email_address'],
+            'permissions_field': []
+        },
+        _expected_status=200,
+    )
+    assert normalize_spaces(page.select_one('span.govuk-error-message').text) == (
+        "Error: You cannot send an invitation to yourself"
+    )
+    assert mock_create_invite.called is False
+
+
 def test_no_permission_manage_users_page(
     client_request,
     service_one,