Add #frozen_string_literal: true to all files

[moberegger]

Simply adds #frozen_string_literal: true to all files. No idea why these were missing. Don't think this will make a huge difference, but it can't hurt.

Are these necessary/useful for Rakefile, Gemfile, etc? This PR includes them, but not 100% sure if that's correct.

We can also consider configuring action_view.frozen_string_literal = true in the railtie. Something like

config.before_configuration do |app|
  # Makes is so that string literals are frozen and interned in our jbuilder templates to save
  # on memory allocations.
  app.config.action_view.frozen_string_literal = true
end

Not sure if this is something that jbuilder should force, but I think this is something you will generally want. Wouldn't surprise me if there are users of the gem that are unaware that strings in their templates aren't being interned.