Upgrade to React 16 + Streaming SSR

[Restuta]

Few things broke as part of this upgrade.

1. opening a modal in a calendar and then reloading the page leads to inconsistent UI
2. React 16 doesn't patch SSR HTML, so when we "guess" a viewport size on the server and render guessed version it stays the same on the client. We need to patch this up with something like Need a way to prevent component render on client side initial load facebook/react#8017 (comment)

see also this suggestion facebook/react#10591 (comment)

TL;DR this means

1. we need to either get rid of this state and just show "normal" event page if page is refreshed, now when we have a "back to calendar" button this should be fine or we just need to render calendar on the client with a delay, e.g. twitter does this and trello
2. to fix this we would need to force re-render of the calendar, there is no other way around this since viewport size is unknown on the server

[Restuta]

It would be nice to start using React Helmet, but it doesn't support streaming yet :( nfl/react-helmet#322

UPDATE:

This lib does https://github.com/staylor/react-helmet-async (https://spectrum.chat/thread/056d029c-99cc-4a54-889c-986497bc5646)