Make YouTube mocked server serve YouTube API

**Description**

1.) Making YouTube mock server serve YouTube API.
2.) Adding ability to block YouTube API from YouTube mock server.
3.) Adding acceptance tests that check YouTube mock server for availability of YouTube API, and for it's ability to block YouTube API.

**Jira issue**

Part of [BLD-939](https://edx-wiki.atlassian.net/browse/BLD-939).

**Reviewers**

- @olmar