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
 		}

app/post.go

@@ -99,7 +99,7 @@ func CreatePost(post *model.Post, teamId string, triggerWebhooks bool) (*model.P
 		rpost = result.Data.(*model.Post)
 	}
 
-	esInterface := einterfaces.GetElasticSearchInterface()
+	esInterface := einterfaces.GetElasticsearchInterface()
 	if esInterface != nil && *utils.Cfg.ElasticSearchSettings.EnableIndexing {
 		go esInterface.IndexPost(rpost, teamId)
 	}
@@ -284,7 +284,7 @@ func UpdatePost(post *model.Post, safeUpdate bool) (*model.Post, *model.AppError
 	} else {
 		rpost := result.Data.(*model.Post)
 
-		esInterface := einterfaces.GetElasticSearchInterface()
+		esInterface := einterfaces.GetElasticsearchInterface()
 		if esInterface != nil && *utils.Cfg.ElasticSearchSettings.EnableIndexing {
 			go func() {
 				if rchannel := <-Srv.Store.Channel().GetForPost(rpost.Id); rchannel.Err != nil {
@@ -471,7 +471,7 @@ func DeletePost(postId string) (*model.Post, *model.AppError) {
 		go DeletePostFiles(post)
 		go DeleteFlaggedPosts(post.Id)
 
-		esInterface := einterfaces.GetElasticSearchInterface()
+		esInterface := einterfaces.GetElasticsearchInterface()
 		if esInterface != nil && *utils.Cfg.ElasticSearchSettings.EnableIndexing {
 			go esInterface.DeletePost(post.Id)
 		}
@@ -502,8 +502,8 @@ func DeletePostFiles(post *model.Post) {
 func SearchPostsInTeam(terms string, userId string, teamId string, isOrSearch bool) (*model.PostList, *model.AppError) {
 	paramsList := model.ParseSearchParams(terms)
 
-	esInterface := einterfaces.GetElasticSearchInterface()
-	if esInterface != nil && *utils.Cfg.ElasticSearchSettings.EnableSearching && utils.IsLicensed && *utils.License.Features.ElasticSearch {
+	esInterface := einterfaces.GetElasticsearchInterface()
+	if esInterface != nil && *utils.Cfg.ElasticSearchSettings.EnableSearching && utils.IsLicensed && *utils.License.Features.Elasticsearch {
 		finalParamsList := []*model.SearchParams{}
 
 		for _, params := range paramsList {
@@ -539,7 +539,7 @@ func SearchPostsInTeam(terms string, userId string, teamId string, isOrSearch bo
 			return nil, err
 		}
 
-		postIds, err := einterfaces.GetElasticSearchInterface().SearchPosts(userChannels, finalParamsList)
+		postIds, err := einterfaces.GetElasticsearchInterface().SearchPosts(userChannels, finalParamsList)
 		if err != nil {
 			return nil, err
 		}