show fetch favicons prompt when dismissing settings

[brindy]

Task/Issue URL: https://app.asana.com/0/392891325557410/1209231808112963/f
Tech Design URL:
CC:

Description:
Adds a notification for settings did disappear and sets a flag when missing icons are detected.

Steps to test this PR:

1. Clean slate app
2. Go to new tab page
3. Open settings and Sync with another device
4. Swipe to close settings
5. Should get prompted to update icons
6. Repeat above steps but back out of settings using back button and done, should also get prompt
7. Smoke test settings (e.g. change some appearance, access email, change the burn animation, etc)
8. Then browse to a page, then create a new tab (tab switcher -> plus button). The prompt should not be shown, because it's already been seen.

Definition of Done (Internal Only):

• Does this PR satisfy our Definition of Done?

[ayoy]

LGTM and works great. Thanks @brindy!