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/songselect.js

@@ -1161,9 +1161,9 @@ class SongSelect{
 				selectedWidth = this.songAsset.selectedWidth
 			}
 			
-			var lastMoveMul = Math.pow(Math.abs(this.state.lastMove), 1 / 4)
+			var lastMoveMul = Math.pow(Math.abs(this.state.lastMove || 0), 1 / 4)
 			var changeSpeed = this.songSelecting.speed * lastMoveMul
-			var resize = changeSpeed * this.songSelecting.resize / lastMoveMul
+			var resize = changeSpeed * (lastMoveMul === 0 ? 0 : this.songSelecting.resize / lastMoveMul)
 			var scrollDelay = changeSpeed * this.songSelecting.scrollDelay
 			var resize2 = changeSpeed - resize
 			var scroll = resize2 - resize - scrollDelay * 2