[BUGFIX] Fix url for youtube embeds

Additionally solve the problem when http:// is missing
due to usage of the link wizard.

Change-Id: I676d14b4ddf81d5d3ec0fc0d0ebb32d08910047a
Resolves: #58569
Resolves: #58986
Releases: 6.2
Reviewed-on: https://review.typo3.org/29952
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers
Reviewed-by: Markus Klein
Tested-by: Markus Klein