This hooks up desktop to the dismissal API in the Rust component [1] and removes
`BlockedSuggestions`.
The Rust dismissal API has two ways to dismiss a suggestion: by `Suggestion`
object and by dismissal key. We use the first one for Rust suggestions and the
second one for other suggestions, like Merino. A dismissal key is just an
arbitrary opaque string token stored in the dismissed-suggestions table in the
Rust component. It's up to consumers (like desktop) to use appropriate dismissal
keys for their non-Rust suggestions.
In order to retain the user's blocked digests that desktop has always recorded
in the `quicksuggest.blockedDigests` pref, I took advantage of dismissal keys by
migrating each digest to a dismissal key in the Rust component.
[1] See bug 1961412 and https://github.com/mozilla/application-services/pull/6714
Differential Revision: https://phabricator.services.mozilla.com/D246369
c6aa8fdd9f