Adds elasticsearch to the user and channel autocompletion functions (#10354)

* Adds elasticsearch to the user and channel autocompletion functions

* Implement channel store GetChannelsByIds test

* Style changes and govet fixes

* Add gofmt fixes

* Extract default channel search limit to a const

* Add StringSliceDiff function to the utils package

* Honor USER_SEARCH_MAX_LIMIT on the user autocomplete api handler

* Change the elasticsearch development image

api4/user.go

@@ -598,6 +598,8 @@ func autocompleteUsers(c *Context, w http.ResponseWriter, r *http.Request) {
 	limit, _ := strconv.Atoi(limitStr)
 	if limitStr == "" {
 		limit = model.USER_SEARCH_DEFAULT_LIMIT
+	} else if limit > model.USER_SEARCH_MAX_LIMIT {
+		limit = model.USER_SEARCH_MAX_LIMIT
 	}
 
 	options := &model.UserSearchOptions{