A React-based replacement for BlazeLayout

[tassoevan]

Proposed changes (including videos or screenshots)

• The Meteor package kadira:blaze-layout was removed;
• A global subscription for the current application layout (appLayout) replaces BlazeLayout entirely;
• The #react-root element is rendered on server-side instead of dynamically injected into the DOM tree;
• The "page loading" throbber is now rendered on the React tree;
• The renderRouteComponent helper was removed;
• Some code run without any criteria on main template module was moved into client startup modules;
• React portals used to embed Blaze templates have their own subscription (blazePortals);
• Some route components were refactored to remove a URL path trap originally disabled by renderRouteComponent;
• A new component to embed the DOM nodes generated by RoomManager was created.

Issue(s)

Steps to test or reproduce

Further comments

What could be the next steps:

1. Convert templates that are not rendered over/under the main template to components (e.g. replacing appLayout.render('xyz') with appLayout.render({ component: lazy(() => import('./Xyz')) }));
2. Render the room using React only first, then optimize it to avoid to many reconciliations between React and the DOM tree;
3. Replace the main template with a new component;
4. Move some initialization from client/startup to effects of the component implemented in (4);
5. Use the URL path as the single source of truth to determine which layout component should be rendered inside AppLayout (like a rendering router that only renders if a route is the current one).