Plugins: Add plugin settings

- Add support for plugin settings, they appear in the same menu as the plugins, indented from the left to emphasize which plugin the setting belongs to
  - Note that plugin settings can still be changed even when the plugins are stopped
- Add tooltips to plugin menu to view the plugin descriptions, description_lang can also be used
- Fix scolling not working on song select when returning from game settings
- Let instance owners set default plugin files in config.py, to make them easier to maintain
- plugins.add() can now add plugins using a url
- Plugins can be hidden from the plugin menu using PluginLoader.hide, an option in plugins.add(), or in config.py
- Make p2.disable() incremental so that multiple plugins can disable multiplayer independently
- Server no longer crashes if certain optional config fields were not copied over from an updated example config
- Fix not being able to unload plugins if one was imported with errors

public/src/js/strings.js

@@ -1323,6 +1323,14 @@ var translations = {
 				one: "%s plugin",
 				other: "%s plugins"
 			}
+		},
+		author: {
+			ja: null,
+			en: "By %s"
+		},
+		version: {
+			ja: null,
+			en: "Version %s"
 		}
 	}
 }