Toolbar selector for calculating height is not very stable

This is related to the toolbar not properly setting the inline style on the body element and then elements being hidden behind the admin toolbar. This is because the selector is not returning an element--for whatever reason, whether it's some conflict with other modules or what--and then calculating the height of the null element returns 0, totaling to a 40px height (when it should be 79px). Instead of expecting all other modules to not step on this module's toes, we can just use a simpler selector for retrieving the height of the admin toolbar.

I've seen this issue come up a few times and it seems like it's always another modules problem. But after looking at the selector for the JS, it seems like it is an unnecessarily complicated selector. We can simply select the '#toolbar-bar' element and find the height from that element. The expectation being that other modules do not reuse the same selector (which is semantically incorrect anyways since they're duplicating HTML IDs at that point).