Use data-ea-placement-bottom to set CSS bottom property

[humitos]

We require a way to tell EthicalAd client to show the ad above the Read the Docs flyout, so they don't collide.

Closes #188

[davidfischer]

This looks good. I added the documentation for it, noted that the value must have a unit, and tested it out. Notably the CSS bottom property doesn't have any effect unless the element is positioned. So this will effect stickybox elements or other custom positioned placement styles but not the default style.

This could have some issues if anybody wants to use CSP but they can use CSS to position it that way if they prefer.

[humitos]

Thanks for adding the documentation. It seems there is an issue with the tests because it can't install Chrome for some reason. Other than that, I think we are ready to move forward with this. Feel free to merge if you consider.

[davidfischer]

Thanks for adding the documentation. It seems there is an issue with the tests because it can't install Chrome for some reason. Other than that, I think we are ready to move forward with this. Feel free to merge if you consider.

The test issue is fixed by #191 so shouldn't be a problem. I'm going to merge it but probably not deploy it until the offsite so we can discuss rolling it out across RTD.