Adding paging to elasticsearch API. (#9425)

einterfaces/elasticsearch.go

@@ -13,7 +13,7 @@ type ElasticsearchInterface interface {
 	Start() *model.AppError
 	Stop() *model.AppError
 	IndexPost(post *model.Post, teamId string) *model.AppError
-	SearchPosts(channels *model.ChannelList, searchParams []*model.SearchParams) ([]string, model.PostSearchMatches, *model.AppError)
+	SearchPosts(channels *model.ChannelList, searchParams []*model.SearchParams, page, perPage int) ([]string, model.PostSearchMatches, *model.AppError)
 	DeletePost(post *model.Post) *model.AppError
 	TestConfig(cfg *model.Config) *model.AppError
 	PurgeIndexes() *model.AppError