config: add debug_mode option.

Add a debug mode option. This is useful when debugging failed tests.

Debug mode disables parallel testing so you can set breakpoints in tests
with binding.pry (normally parallel testing makes it hard to set
breakpoints).

Debug mode also disables global exception handling for controllers. This
lets exceptions bubble up to the console during controller tests
(normally exceptions are swallowed by the controller, which prevents you
from seeing backtraces in failed controller tests).

app/controllers/application_controller.rb

@@ -88,6 +88,8 @@ class ApplicationController < ActionController::Base
   end
 
   def rescue_exception(exception)
+    return if Danbooru.config.debug_mode
+
     case exception
     when ActionView::Template::Error
       rescue_exception(exception.cause)