(core) Replace questionnaire for new users with a popup asking for just their primary use-case.

Summary:
- WelcomeQuestions implements the new popup.
- Popup shows up on any doc-list page, the first time the user visits one after
  signing up and setting their name.
- Submits responses to the same "New User Questions" doc, which has been
  changed to accept two new columns (ChoiceList of use_cases, and Text for
  use_other).
- Improve modals on mobile along the way.

Test Plan: Added browser tests and tested manually

Reviewers: alexmojaki

Reviewed By: alexmojaki

Subscribers: jarek

Differential Revision: https://phab.getgrist.com/D3213

app/common/Prefs.ts

@@ -6,14 +6,18 @@ export type SortPref = typeof SortPref.type;
 export const ViewPref = StringUnion("list", "icons");
 export type ViewPref = typeof ViewPref.type;
 
-
 // A collection of preferences related to a user or org (or combination).
 export interface Prefs {
-  // TODO replace this with real preferences.
+  // A dummy field used only in tests.
   placeholder?: string;
 }
 
-export type UserPrefs = Prefs;
+// A collection of preferences related to a user or org (or combination).
+export interface UserPrefs extends Prefs {
+  // Whether to ask the user to fill out a form about their use-case, on opening the DocMenu page.
+  // Set to true on first login, then reset when the form is closed, so that it only shows once.
+  showNewUserQuestions?: boolean;
+}
 
 export interface UserOrgPrefs extends Prefs {
   docMenuSort?: SortPref;