Assorted fixes and updates to HTML New Tab Page

[ayoy]

Task/Issue URL: https://app.asana.com/0/72649045549333/1209186302756177/f

Description:
This change contains updates to HTML New Tab Page: fixed displaying favicons fetcher
onboarding dialog, added user image background thumbnails context menu
and updated favicon placeholder to 2 letters.

Steps to test this PR:

1. I've had favicons fetcher onboarding dialog tested and verified already (see this comment in Asana).
2. Open the app, set internal user state and enable htmlNewTabPage feature flag.
3. Import some bookmarks and verify that favorites placeholders in New Tab Page display 2 letters.
4. Set image as background, go to customize panel -> My Backgrounds, verify that image thumbnails have context menu that allows deleting images and that deleting via context menu works.

Definition of Done:

• Does this PR satisfy our Definition of Done?

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