[ticket/10155] Update jQuery to 1.8.3

Currently included jQuery is old by now, v1.6.2. We should update to
1.8.3. This allows us to take advantage of the latest form of jQuery
event delegation ($.on). I don't think it wise to update to jQuery
1.9.x yet, as many 3rd party scripts still need to be updated to cope
with its deprecated features ($.browser). Therefor, 1.8.3 is the latest
and most widely compatible stable version right now.

PHPBB3-10155