Add material-components-web-components as a submodule
The material web components (https://github.com/material-components/material-components-web-components) elements are used as a base for the lh-toolkit-webcomponents. Many of their custom elements have not been released to NPM registry yet. Thus, their repo needs to be added as a submodule for us to track and update. The submodule will be removed once all the custom elements based on material design has been release. Till then we will use a clone of their repo.