support overriding application owner with config option

[ericdude4]

Currently, the application_owner config option is ignored. Referenced in docs: https://hexdocs.pm/ex_oauth2_provider/ExOauth2Provider.html#module-configuration

Issue #67

[danschultzer]

Thanks! I was thinking about this and prefer instead to require an explicit override like this:

defmodule MyApp.OauthApplications.OauthApplication do
  use Ecto.Schema
  use ExOauth2Provider.Applications.Application

  schema "oauth_applications" do
    belongs_to :owner, MyApp.Users.User

    application_fields()

    timestamps()
  end
end

This would also prevent compile time issues with config.