This adds preview localizations for remaining Firefox Suggest strings.
I removed support for `payload.helpTitle` and `sponsoredText`. The `helpTitle`
code comment says it's useful for experiments with hardcoded strings, but we're
not shipping experiments as extensions anymore, and in-tree experimental/
in-development features should use preview localizations AFAICT.
Covered by existing tests:
* browser/components/urlbar/tests/browser/browser_helpUrl.js
* browser/components/urlbar/tests/browser/browser_quickSuggest*
* browser/components/preferences/tests/browser_searchQuickSuggest.js
Depends on D122550
Differential Revision: https://phabricator.services.mozilla.com/D123032
f959ed5916