(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

static/ui-icons/use-cases/UseEducate.svg

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