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.

i18n/en.json

@@ -3491,6 +3491,18 @@
     "id": "ent.compliance.run_started.info",
     "translation": "Compliance export started for job '{{.JobName}}' at '{{.FilePath}}'"
   },
+  {
+    "id": "ent.elasticsearch.delete_post.error",
+    "translation": "Failed to delete the post"
+  },
+  {
+    "id": "ent.elasticsearch.index_post.error",
+    "translation": "Failed to index the post"
+  },
+  {
+    "id": "ent.elasticsearch.not_started.error",
+    "translation": "Elasticsearch is not started"
+  },
   {
     "id": "ent.elasticsearch.search_posts.disabled",
     "translation": "ElasticSearch searching is disabled on this server"
@@ -3523,6 +3535,10 @@
     "id": "ent.elasticsearch.start.index_settings_failed",
     "translation": "Failed to set ElasticSearch index settings"
   },
+  {
+    "id": "ent.elasticsearch.test_config.connect_failed",
+    "translation": "Connecting to Elasticsearch server failed."
+  },
   {
     "id": "ent.elasticsearch.test_config.indexing_disabled.error",
     "translation": "Elasticsearch is disabled."