Standardise assets configuration

[dylanwolff]

I ran into an issue with asset configuration when setting up a staging app on Heroku today. The stacktrace pointed me to the #read method in system/app_prototype/assets:

def read(asset)
  path = self[asset]

  if File.exist?(path)
    File.read(path)
  else
    open(path, "r:UTF-8").read
  end
end

I was able to fix the issue, but noticed we're using a slightly different config in other recent apps (eg MUP):

def read(asset)
  path = self[asset]
  uri = begin
    URI(path)
  rescue URI::InvalidURIError
    nil
  end

  if uri&.absolute?
    open(uri.to_s, "r:UTF-8").read
  elsif File.exist?("#{root}/public#{path}")
    File.read("#{root}/public#{path}")
  else
    raise ArgumentError, "asset not found"
  end
end

@makenosound mentioned we should look at the setup of this asset class in general so I'm opening this one up so we can discuss.