(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/client/ui2018/buttons.ts

@@ -39,7 +39,8 @@ export const cssButton = styled('button', `
 
   &-large {
     font-weight: 500;
-    padding: 12px 24px;
+    padding: 10px 24px;
+    min-height: 40px;
   }
 
   &-primary {