Create a SearchMiddleware for communicating with the storage

[pocmo]

RFC: #7655

A Middleware will be responsible for listening to actions that modify the SearchState (#7861) and perform the same actions on the persistent storage: If a custom search engine is added then the middleware will save it to disk too. If a custom search engine is removed then the middleware will remove it from the storage too. In addition to that the middleware will also be responsible for loading the initial list of search engines and dispatching an action on the store to add them to the state.

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