Fix BBCCoUkIPlayerPlaylistIE

[dirkf]

Please follow the guide below

• You will be asked some questions, please read them carefully and answer honestly
• Put an x into all the boxes [ ] relevant to your pull request (like that [x])
• Use Preview tab to see how your pull request will actually look like

---

Before submitting a pull request make sure you have:

• Searched the bugtracker for similar pull requests
• Read adding new extractor tutorial
• Read youtube-dl coding conventions and adjusted the code to meet them
• Covered the code with tests (note that PRs without tests will be REJECTED)
• Checked the code with flake8

In order to be accepted and merged into youtube-dl each piece of code must be in public domain or released under Unlicense. Check one of the following options:

• I am the original author of this code and I am willing to release it under Unlicense
• I am not the original author of this code but it is in public domain or released under Unlicense (provide reliable evidence)

What is the purpose of your pull request?

• Bug fix
• Improvement
• New extractor
• New feature

---

Description of your pull request and other information

The BBCCoUkIPlayerPlaylistIE extractor was failing, whereas BBCCoUkPlaylistIE still worked for the same pids: eg, the _TESTS.

In the IPlayer series pages, the playlist metadata is now sent in a JSON expression within a <script> element, and this patch extracts the title and description from it, and modifies the _VIDEO_ID_TEMPLATE to match the episode links within the entities subobject of the JSON.