Avoid passing global state in admin_actions.jsx in Mattermost webapp

Description

This ticket is regarding the way we handle state actions in webapp. Similar to the util functions, we are passing the state and dispatch directly from our imported global store in the action creators.

In order to make the action creators more predictable and improve their testability we need to refactor the way we handle action creators and make sure they are always passed to mapDispatchToProps in the connected components.

Files: admin_actions.jsx

Proposed change in post_actions.jsx:

// get the state from getState
  ......
  dispatch({
            type: SearchTypes.RECEIVED*SEARCH*POSTS,
            data: {posts, order: newResults},
        });
} 

and then we call it dispatch(addPostToSearchResults(postId));

NOTE: Calling getState is fine as long as it's coming from thunk inside the component. We don't want to import it from stores/redux_store

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JIRA: https://mattermost.atlassian.net/browse/MM-47431