center the help dialog in the middle of the page
center the help dialog in the middle of the page

fixes: CNVS-31922

when I was auditing all of the cases where people 
were using RequireJS as we're getting ready to turn 
on webpack I noticed that there were a few that were 
manually hacking our help dialog code to recenter 
the modal into the middle of the page after all the 
links are loaded. that is just a bug that we don't 
do that and we should just do it ourselves, then 
they wouldn't have to hack around it in their custom JS.


This was the snippet I saw in people's custom JS to do it:
// this is a lot of code but basically it's just so 
// that the Help dialog is centered on the first click,
// instead of appearing to be dragged down lower. 
// Hopefully Canvas fixes this themselves and we remove it
    require(["compiled/helpDialog"], function(help){
        help.initTriggers().click(function(){
            help.helpLinksDfd.done(function(){
                var widget = $(help.$dialog).dialog('widget');

                widget.center();
            });
        });
    });

test plan:
* from the login page, click the "help" link
* the modal that appears should be in the middle
  of the window, and not slightly towards the bottom

Change-Id: I7815d1a4a10d4855bdf8819ee76efd7530d8cc6f
Reviewed-on: https://gerrit.instructure.com/90352
Tested-by: Jenkins
Reviewed-by: John Corrigan <3d73713d0cc718326e4fd7c06c64e067ad6e2270@instructure.com>
QA-Review: August Thornton <9adc7a1161ddf32ff608de792a7e50179545f026@instructure.com>
Product-Review: Ryan Shaw <ea3cd978650417470535f3a4725b6b5042a6ab59@instructure.com>

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