[youtube] Fix videos with age gate and encrypted signatures

The `sts` value is available on the embed webpage, get it from there.

Fixes #4108.
[daum] update 'full id' regex

fixes #4566
Merge pull request #4599 from t0mm0/daum_fix

[daum] update 'full id' regex
[daum] Improve full_id regex
Merge pull request #4598 from 0xced/encrypted-age-gate

[youtube] Fix videos with age gate and encrypted signatures
Credit @0xced for #4598
[youtube] Add test for age-gate video with encrypted signature