Restore concurrent uploading functionality.

The preference for concurrent uploads was broken in two ways:

1) No default option was specified via $wgDefaultUserOptions.
   This meant that until the preference was set, it would not
   work.

2) In turn, the 'default' property of the preferences array was
   misused. From Preferences.php:

   "Note that the 'default' field is named for generic forms, and does
    not represent the preference's default (which is stored in
    $wgDefaultUserOptions), but the default for the form field, which
    should be whatever the user has set for that preference. There
    is no need to override it .."

Bug: 48090
Change-Id: I0d9f13c6b7698056d4f9ed113b9b23b2f266263d
Merge "Restore concurrent uploading functionality."