- Convert the <simple-search> directive to a modern component-style
   directive which uses one-way bindings and events rather than two-way
   bindings. This makes it consistent with other components in the app.

 - Avoid duplicating the search filter state in multiple places but
   store it only in the Redux store and update it via an action.

The tests for `simple-search` have not been updated in this commit
because they are going to be replaced with a new set using the
`createDirective()` helper in the next commit.