[React App]: Add email subscription page UI

[vovakulikov]

Notion PRD https://www.notion.so/sourcegraph/Communication-preferences-PRD-45ff434ff698482f971ec0c487943634

This PR (at the moment) just adds a new page with communication preferences form UI.

TODO

• Add basic form UI page with form designs
• Add logic that this page shows automatically only when the user sign-in for the first time
• Connects it to the designated API endpoint to save preferences on the backend
• Add successful state UI after the user clicks the next button

Test plan

• Run a fresh (with no temporary settings) instance of Sourcegraph locally
• Try to sing in
• Check that after successful sign-in you see the Communication preferences page
• Submit the form, after submitting you should be redirected to the main or any other page if it's presented in returnTo URL query param