(core) Update design of empty docs home page, and add a "Manage Team" button.

Summary:
- Remove the empty-folder icon
- Add an "Invite team members" button for owners on empty team sites
- Add a "Browse Templates" button for all other cases on empty sites
- Update intro text for team, including a link to Sprouts
- Update intro text for personal/anon.
- Include a Free/Pro tag for team sites (for now, only "Free")
- Add a "Manage Team" button for owners on home page of all team sites.
- Polished the UI of UserManager: add a transition for the background, and
  delay the appearance of the spinner for fast loads.

Test Plan: Fixed up the HomeIntro tests; added test case for Manage Team button

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D3459

app/common/gristUrls.ts

@@ -60,6 +60,7 @@ export const commonUrls = {
   help: "https://support.getgrist.com",
   plans: "https://www.getgrist.com/pricing",
   createTeamSite: "https://www.getgrist.com/create-team-site",
+  sproutsProgram: "https://www.getgrist.com/sprouts-program",
 
   efcrConnect: 'https://efc-r.com/connect',
   efcrHelp: 'https://www.nioxus.info/eFCR-Help',