PLT-6472: Basic Elastic Search implementation. (#6382)

* PLT-6472: Basic Elastic Search implementation.

This currently supports indexing of posts at create/update/delete time.
It does not support batch indexing or reindexing, and does not support
any entities other than posts yet. The purpose is to more-or-less
replicate the existing full-text search feature but with some of the
immediate benefits of using elastic search.

* Alter settings for AWS compatability.

* Remove unneeded i18n strings.

i18n/en.json

@@ -3307,6 +3307,38 @@
     "id": "ent.compliance.run_started.info",
     "translation": "Compliance export started for job '{{.JobName}}' at '{{.FilePath}}'"
   },
+  {
+    "id": "ent.elasticsearch.start.connect_failed",
+    "translation": "Setting up ElasticSearch Client Failed"
+  },
+  {
+    "id": "ent.elasticsearch.start.index_exists_failed",
+    "translation": "Failed to establish whether ElasticSearch index exists"
+  },
+  {
+    "id": "ent.elasticsearch.start.index_create_failed",
+    "translation": "Failed to create ElasticSearch index"
+  },
+  {
+    "id": "ent.elasticsearch.start.index_settings_failed",
+    "translation": "Failed to set ElasticSearch index settings"
+  },
+  {
+    "id": "ent.elasticsearch.start.index_mapping_failed",
+    "translation": "Failed to setup ElasticSearch index mapping"
+  },
+  {
+    "id": "ent.elasticsearch.search_posts.disabled",
+    "translation": "ElasticSearch searching is disabled on this server"
+  },
+  {
+    "id": "ent.elasticsearch.search_posts.search_failed",
+    "translation": "Search failed to complete"
+  },
+  {
+    "id": "ent.elasticsearch.search_posts.unmarshall_post_failed",
+    "translation": "Failed to decode search results"
+  },
   {
     "id": "ent.emoji.licence_disable.app_error",
     "translation": "Custom emoji restrictions disabled by current license. Please contact your system administrator about upgrading your enterprise license."
@@ -3859,6 +3891,22 @@
     "id": "model.compliance.is_valid.start_end_at.app_error",
     "translation": "To must be greater than From"
   },
+  {
+    "id": "model.config.is_valid.elastic_search.connection_url.app_error",
+    "translation": "Elastic Search ConnectionUrl setting must be provided when Elastic Search indexing is enabled."
+  },
+  {
+    "id": "model.config.is_valid.elastic_search.username.app_error",
+    "translation": "Elastic Search Username setting must be provided when Elastic Search indexing is enabled."
+  },
+  {
+    "id": "model.config.is_valid.elastic_search.password.app_error",
+    "translation": "Elastic Search Password setting must be provided when Elastic Search indexing is enabled."
+  },
+  {
+    "id": "model.config.is_valid.elastic_search.enable_searching.app_error",
+    "translation": "Elastic Search IndexingEnabled setting must be set to true when Elastic Search SearchEnabled is set to true."
+  },
   {
     "id": "model.config.is_valid.cluster_email_batching.app_error",
     "translation": "Unable to enable email batching when clustering is enabled."
@@ -5179,6 +5227,10 @@
     "id": "store.sql_post.get.app_error",
     "translation": "We couldn't get the post"
   },
+  {
+    "id": "store.sql_post.get_posts_by_ids.app_error",
+    "translation": "We couldn't get the posts"
+  },
   {
     "id": "store.sql_post.get_parents_posts.app_error",
     "translation": "We couldn't get the parent post for the channel"