Removed z-index from flash container

## What does this MR do?

Fixes a styling bug where the flash container would be above the header when scrolling.

Before it looked like this when scrolling:

![Screen_Shot_2016-09-08_at_14.06.36](/uploads/d639287329643d26454a997e3917bc53/Screen_Shot_2016-09-08_at_14.06.36.png)

Now it looks like this when scrolling:

![Screen_Shot_2016-09-08_at_14.07.37](/uploads/7c1c5e38adc9b9011558d4190d3520fe/Screen_Shot_2016-09-08_at_14.07.37.png)

## Are there points in the code the reviewer needs to double check?

I can't think of any good reason why this would need to be above another element. But you never know!

Closes #20713 

See merge request !6264