[tv2] Add support for mtvuutiset.fi

[tpikonen]

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

What is the purpose of your pull request?

• New extractor

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Description of your pull request and other information

Extracts videos from articles / web pages in mtvuutiset.fi. The videos share the backend with mtv.fi / katsomo.fi / tv2.no.

An improved resubmission of #26658 with a live branch.