Bug 21742: Fix YouTube variable propagation

Bug 21742: Fix YouTube variable propagation

The loop to find all media has an $isyoutube variable. This was declared
outside the loop and never reset. Once a YouTube video was encountered
all subsequent videos were considered as YouTube. This patch reduces the
scope of the variable and resets per loop.

To test:
1 - Enable HTML5Media and HTML5MediaYouTube
2 - Add 856$u to record for a youtube vido and a pdf (sample record
        attached to bug)
3 - View the record, the Play media tab has two video boxes
4 - Play the second, it fails
5 - Apply patch
6 - View record, Play media should include only one video link

Signed-off-by: Devinim <kohadevinim@devinim.com.tr>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>