Runtime configuration

Related to #85

Everything should now be configured at runtime, with the exception of
the `Pleroma.HTML` scrubbers (the scrubbers used can be
changed at runtime, but their configuration is compile-time) because
it's building a module with a macro.

lib/pleroma/formatter.ex

@@ -29,8 +29,6 @@ defmodule Pleroma.Formatter do
     |> Enum.filter(fn {_match, user} -> user end)
   end
 
-  @instance Application.get_env(:pleroma, :instance)
-
   def emojify(text) do
     emojify(text, Emoji.get_all())
   end