In-App Youtube Player

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      1. Description
         Added support of in-app YouTube player.

<img src="https://user-images.githubusercontent.com/32511718/42520530-e28cf836-847f-11e8-9dfb-864084ed6b70.png" width="300"> <img src="https://user-images.githubusercontent.com/32511718/42520570-ffcc5c0c-847f-11e8-8a89-2892118227cb.png" width="300">

<img src="https://user-images.githubusercontent.com/32511718/42520487-cc5e544c-847f-11e8-9915-c39bb464f20d.png" width="200">

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      1. Feature flag
         This feature is behind a flag namely "INAPP_YOUTUBE_PLAYER". Sample flag model is as below:

```
INAPP_YOUTUBE_PLAYER:
ENABLED: true
YOUTUBE_PLAYER_KEY: 'XYZXYZ'
```

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      1. Notes
         Altered video playing order in CourseUnitPagerAdapter. Previously the adapter checked and rendered the related fragments for units in following order:
         1. CourseUnitVideo
         2. CourseUnitOnlyOnYoutube
         3. CourseUnitDiscussion
         4. CourseUnitMobileNotSupported
         5. CourseUnitEmpty
         6. CourseUnitWebView
         7. CourseUnitMobileNotSupported

I have changed the above order to the following:
1. CourseUnitOnlyOnYoutubeNotSupported
2. CourseUnitOnlyOnYoutube
3. CourseUnitVideo
4. CourseUnitDiscussion
5. CourseUnitMobileNotSupported
6. CourseUnitEmpty
7. CourseUnitWebView
8. CourseUnitMobileNotSupported

The feature flag will be checked first to see if we can play YouTube video within app. If not, then the `CourseUnitOnlyOnYoutubeNotSupported` fragment will be loaded where user will be asked to view the video in the browser or YouTube app. Then if the video is already downloaded, it will be played as `CourseUnitVideo` rather than as YouTube video.

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      1. Docs

• [Overview](https://developers.google.com/youtube/android/player/)

• [YouTubePlayerSupportFragment](https://developers.google.com/youtube/android/player/reference/com/google/android/youtube/player/YouTubePlayerSupportFragment)

• [Sample](https://developers.google.com/youtube/android/player/sample-applications)

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      1. Testing

• [ ] i18n

• [ ] RTL

• [ ] Performance