Metatag Asynchronous Widget

The Metatag widget can affect content editing performance especially when Schema Metatag is enabled. The widget provided by the Metatag module is not called MetatagFirehose for nothing! This module provides a new widget that only loads the metatag widget only when required.

Roadmap

* #3203950: Create a plan to improve widget UX The initial implementation solves the performance problem for editing the node when you don't need to edit a meta tag. In some ways the meta tag editing experience has regressed because in order to edit/view you need to click an extra button.

Thanks to people who worked on #3057523: Content forms are painfully slow if multiple sub-modules are enabled - especially @Berdir who provided code to jump start this project.

The reason for yet-another-module is to get something usable and easy to install & maintain without requiring a quicker release cadence for the Metatag module. If, once the module is feature complete, the maintainers of the Metatag module want to include the widget in the Metatag module itself, then this module will be deprecated.