(core) Product update popups and hosted stripe integration

Summary:
- Showing nudge to individual users to sign up for free team plan.
- Implementing billing page to upgrade from free team to pro.
- New modal with upgrade options and free team site signup.
- Integrating Stripe-hosted UI for checkout and plan management.

Test Plan: updated tests

Reviewers: georgegevoian

Reviewed By: georgegevoian

Subscribers: paulfitz

Differential Revision: https://phab.getgrist.com/D3456

app/client/ui2018/IconList.ts

@@ -100,6 +100,7 @@ export type IconName = "ChartArea" |
   "Settings" |
   "Share" |
   "Sort" |
+  "Sparks" |
   "Tick" |
   "TickSolid" |
   "Undo" |
@@ -222,6 +223,7 @@ export const IconList: IconName[] = ["ChartArea",
   "Settings",
   "Share",
   "Sort",
+  "Sparks",
   "Tick",
   "TickSolid",
   "Undo",