The dashboard on /admin/ crashed when a brand new widget was added

to the page. The expected output is that the new widget, whether 

it's a primary or secondary widget, should appear on the dashboard 

with the other widgets. 
This was because there was a check to find the new widgets position,

before it actually had been given one. This raised a KeyError,

which crashed the page.
I added some .get() checks, so that if the widget position is not in

the dictionary, a None value is returned.
I also refactored code on how new widgets are rendered in the 

dashboard, so that the dashboard() function in admin/views.py

is much easier to read.

I ran all tests using

./tests/runtests.py and no errors resulted.
I also used the JS-tests on /js-tests/, and there were no issues. 
I created a Unit Test class named WidgetTests, that can be run

with ./tests/runtests.py reviewboard.admin.tests:WidgetTests.

This test checks that adding a new primary widget and a new secondary

widget on the dashboard does not raise a KeyError, and gives the two

widgets a position.