Images uploaded with Thesis aren't cached properly.

[silasjmatson]

I'm not fully sure this is a thesis issue, but it needs more investigation.

Images were being served with Cache-Control:max-age=0, private, must-revalidate

We should either allow the Cache-Control header to be configured w/in thesis, or serve them with a Cache-Control header similar to normal assets, or set a reasonable max-age value.

[silasjmatson]

One solution would be to set the 'Cache-Control' headers here: https://github.com/infinitered/thesis-phoenix/blob/master/lib/thesis/api_controller.ex#L57-L61