[Kaltura] Add support for an uncommon typo in a GDC Vault page

[zorbathut]

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Description of your pull request and other information

A few pages on the GDC Vault (https://www.gdcvault.com/play/1025699, https://www.gdcvault.com/play/1025739, possibly more) have a slightly-misformed Kaltura iframe link, where the URL has an unnecessary leading space. The Kaltura extractor isn't durable enough to ignore that so it fails to download the file. Web browsers seem happy to skip it, however.

This allows it to ignore that initial leading space when it exists.

This is a repost of #21079 which got kinda nuked in the whole DMCA thing.

[dstftw]

Does not seem to have any effect:

> py -3.7 .\youtube_dl\__main__.py https://www.gdcvault.com/play/1025699 -v
[debug] System config: []
[debug] User config: []
[debug] Custom config: []
[debug] Command-line args: ['https://www.gdcvault.com/play/1025699', '-v']
[debug] Encodings: locale cp1251, fs utf-8, out utf-8, pref cp1251
[debug] youtube-dl version 2021.04.26
[debug] Git HEAD: 61d7f7d
[debug] Python version 3.7.0 (CPython) - Windows-10-10.0.10240-SP0
[debug] exe versions: ffmpeg N-85653-gb4330a0, ffprobe N-85653-gb4330a0, phantomjs 2.1.1, rtmpdump 2.4
[debug] Proxy map: {}
[GDCVault] 1025699: Downloading webpage
WARNING: [GDCVault] It looks like http://www.gdcvault.com/play/1025699 requires a login. Try specifying a username and password and try again.
WARNING: [GDCVault] Could not login.
ERROR: Unable to extract xml filename; please report this issue on https://yt-dl.org/bug . Make sure you are using the latest version; see  https://yt-dl.org/update  on how to update. Be sure to call youtube-dl with the --verbose flag and include its complete output.

Add: it's because kaltura alternatives didn't work.

[zorbathut]

You'll need a GDC Vault login to test it, unfortunately; it's joining a similar group because very few of the tests in there still work.

[dstftw]

This has nothing to do with GDC Vault login. The video is freely available.