WIP: Add distillery for producing binary releases

Distillery is a means of producing binary releases of elixir projects, which can be run without requiring an external erlang runtime.

This MR attempts to introduce distillery to pleroma, permitting the use of mix release to build a binary release.

Without !141 (closed) , the code does not function.

Without some form of runtime configuration, the code also does not function. I've introduced an example config.prod.exs which loads things from envvars, but I don't think it's acceptable (or even works?) in its current form.

The two factors above mean this must be WIP for now.