PLT-6595: API to purge Elasticsearch indexes. (#6971)

einterfaces/elasticsearch.go

@@ -11,6 +11,7 @@ type ElasticsearchInterface interface {
 	SearchPosts(channels *model.ChannelList, searchParams []*model.SearchParams) ([]string, *model.AppError)
 	DeletePost(postId string) *model.AppError
 	TestConfig(cfg *model.Config) *model.AppError
+	PurgeIndexes() *model.AppError
 }
 
 var theElasticsearchInterface ElasticsearchInterface