Valera/add timeout china

**Description**

In countries where [youtube.com](http://www.youtube.com) is blocked, sometimes the students have to wait a long time before the page loads. This is because the [YouTube API](https://developers.google.com/youtube/js_api_reference) is loaded by a simple `<script />` tag which causes the browser to halt all execution and wait for the script to load before continuing with fetching other JavaScript files, and rendering the DOM to the page.

**Solution**

The first time a page loads that has a YouTube API included, it tries to load the API. After some time, if nothing loads, or an error is returned, then the back-end will be notified that the user doesn't have YouTube available for them. A setting on the server will be stored. On next page load, the loading of the YouTube API will be skipped.

After the page loads, an asynchronous request for the YouTube API will be made. If it is determined that YouTube API is once more available for the student, then the back-end will be notified that the student has access to the YouTube API. The appropriate setting will be updated, and the student will have YouTube API load on subsequent page loads.

**Technical details**

The YouTube API is loaded from 3 points:

1. The [Studio template](https://github.com/edx/edx-platform/blob/master/cms/templates/base.html) loads the API as a dependency via the Require JS interface.
2. The [LMS template](https://github.com/edx/edx-platform/blob/master/lms/templates/courseware/courseware.html) loads the API as a simple `<script />` tag.
3. The [OVA](http://openvideoannotation.org/) vendor JavaScript module [loads the API](https://github.com/edx/edx-platform/blob/master/common/static/js/vendor/ova/vjs.youtube.js) by programmatically inserting a script tag. Introduced in commit [7e2652b5a8ddaee31594634667293e0fa8967b31](https://github.com/edx/edx-platform/commit/7e2652b5a8ddaee31594634667293e0fa8967b31).

**YouTube blocked imitation**

To simulate the effect of an inaccessible YouTube, you can clear your browser's cache, and add the following to your `/etc/hosts` file:

    127.0.0.1       youtube.com
    127.0.0.1       www.youtube.com
    127.0.0.1       173.194.70.93

*NOTE: Make sure that you don't have a web server running locally (listening and responding on port 80)!*

**JIRA issue**

[BLD-531](https://edx-wiki.atlassian.net/browse/BLD-531)

**Original pull request that was closed by mistake**

[2952](https://github.com/edx/edx-platform/pull/2952)