sylph

Also includes background_image on action pages

Hi @rgrmrts ! I was planning on working on this in the next few weeks -- I'm anticipating that most of the issues will be with the docker image, which...

this definitely breaks rubocop, fixing

given that [travis-ci.org](https://travis-ci.community/t/builds-hang-in-queued-state/10250) isn't running at full capacity, I'm not super concerned about the push build failing, but I'll look into it a little

We should be able to use [apparition](https://github.com/twalpole/apparition) instead of poltergeist!

we should also use the slim version of the image base

rspec is segfaulting locally for me right now, looking into it

This is a bug for previews on brand new lessons! the broken code is likely to be in https://github.com/EFForg/active_preview , not the SEC code base