(core) Add behavioral and coaching call popups

Summary:
Adds a new category of popups that are shown dynamically when
certain parts of the UI are first rendered, and a free coaching
call popup that's shown to users on their site home page.

Test Plan: Browser tests.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3706

stubs/app/client/ui/WelcomeCoachingCall.ts

@@ -0,0 +1 @@
+export * from 'app/client/ui/WelcomeCoachingCallStub';