[BUGFIX] Extension manager page browser not disabled on list update

The extension list on "get extensions" only masks the table when
loading where it should also mask the page browser as you will get
errors when using it while the list is still loading.

The mask now covers the whole ter table content.

Change-Id: I404e4f1a5b9b62a53392491c911d654ce6f007b3
Releases: 6.2, 6.3
Fixes: #55317
Reviewed-on: http://review.typo3.org/31824
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>