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.

Makefile

@@ -123,6 +123,14 @@ ifeq ($(BUILD_ENTERPRISE_READY),true)
 		docker start mattermost-openldap > /dev/null; \
 		sleep 10; \
 	fi
+
+	@if [ $(shell docker ps -a | grep -ci mattermost-elasticsearch) -eq 0 ]; then \
+		echo starting mattermost-elasticsearch; \
+		docker run --name mattermost-elasticsearch -p 9200:9200 -e "http.host=0.0.0.0" -e "transport.host=127.0.0.1" -e "ES_JAVA_OPTS=-Xms250m -Xmx250m" -d grundleborg/elasticsearch:latest > /dev/null; \
+	elif [ $(shell docker ps | grep -ci mattermost-elasticsearch) -eq 0 ]; then \
+		echo restarting mattermost-elasticsearch; \
+		docker start mattermost-elasticsearch> /dev/null; \
+	fi
 endif
 
 stop-docker:
@@ -148,6 +156,11 @@ stop-docker:
 		docker stop mattermost-inbucket > /dev/null; \
 	fi
 
+	@if [ $(shell docker ps -a | grep -ci mattermost-elasticsearch) -eq 1 ]; then \
+		echo stopping mattermost-elasticsearch; \
+		docker stop mattermost-elasticsearch > /dev/null; \
+	fi
+
 clean-docker:
 	@echo Removing docker containers
 
@@ -175,6 +188,12 @@ clean-docker:
 		docker rm -v mattermost-inbucket > /dev/null; \
 	fi
 
+	@if [ $(shell docker ps -a | grep -ci mattermost-elasticsearch) -eq 1 ]; then \
+		echo removing mattermost-elasticsearch; \
+		docker stop mattermost-elasticsearch > /dev/null; \
+		docker rm -v mattermost-elasticsearch > /dev/null; \
+	fi
+
 check-client-style:
 	@echo Checking client style