PLT-6976: Elasticsearch capitalisation and tests. (#6839)

* Fixes Elasticsearch to have consistent capitalisation everywhere
  across the code and UI (except the config file unfortunately).
* Adds basic unit tests for Elastichsearch.
* Adds a Elasticsearch docker image to the Makefile to enable testing
  the Elasticsearch feature.

app/elasticsearch.go

@@ -11,7 +11,7 @@ import (
 )
 
 func TestElasticsearch() *model.AppError {
-	if esI := einterfaces.GetElasticSearchInterface(); esI != nil {
+	if esI := einterfaces.GetElasticsearchInterface(); esI != nil {
 		if err := esI.TestConfig(); err != nil {
 			return err
 		}