Restructure govuk-alerts endpoint to be internal

In response to: https://github.com/alphagov/notifications-api/pull/3305#pullrequestreview-726672421

Previously this was added among the public /v2 endpoints, but it's
only meant for internal use. While only the govuk-alerts app would
be able to access it, the location and /v2 URL suggested otherwise.
This restructures the endpoint so it resembles other internal ones.

tests/app/v2/govuk_alerts/test_get_broadcasts.py

@@ -1,39 +0,0 @@
-from datetime import datetime
-
-from flask import current_app, json
-
-from app.models import BROADCAST_TYPE
-from tests import create_internal_authorization_header
-from tests.app.db import create_broadcast_message, create_template
-
-
-def test_get_all_broadcasts_returns_list_of_broadcasts_and_200(
-    client, sample_broadcast_service
-):
-    template_1 = create_template(sample_broadcast_service, BROADCAST_TYPE)
-
-    broadcast_message_1 = create_broadcast_message(
-        template_1,
-        starts_at=datetime(2021, 6, 15, 12, 0, 0),
-        status='cancelled')
-
-    broadcast_message_2 = create_broadcast_message(
-        template_1,
-        starts_at=datetime(2021, 6, 22, 12, 0, 0),
-        status='broadcasting')
-
-    jwt_client_id = current_app.config['GOVUK_ALERTS_CLIENT_ID']
-    header = create_internal_authorization_header(jwt_client_id)
-
-    response = client.get('/v2/govuk-alerts', headers=[header])
-
-    json_response = json.loads(response.get_data(as_text=True))
-
-    assert response.status_code == 200
-    assert len(json_response['alerts']) == 2
-
-    assert json_response['alerts'][0]['id'] == str(broadcast_message_2.id)
-    assert json_response['alerts'][0]['starts_at'] == '2021-06-22T12:00:00.000000Z'
-    assert json_response['alerts'][0]['finishes_at'] is None
-    assert json_response['alerts'][1]['id'] == str(broadcast_message_1.id)
-    assert json_response['alerts'][1]['starts_at'] == '2021-06-15T12:00:00.000000Z'