Switch backend specs to use poltergeist

:ghost:

Using poltergeist should (hopefully) be faster, more consistent, and more convenient for developers (no need for Xvfb).

For the most part, the changes needed were minimal and made the specs better. specs should all still run properly under selenium.

In a (unscientific) test on my work desktop, this speed up `rspec` in backend from ~4m30s to ~3m30s