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.

config/config.json

@@ -46,6 +46,14 @@
         "EnableUserStatuses": true,
         "ClusterLogTimeoutMilliseconds": 2000
     },
+    "ElasticSearchSettings": {
+        "ConnectionUrl": "http://dockerhost:9200",
+        "Username": "elastic",
+        "Password": "changeme",
+        "EnableIndexing": false,
+        "EnableSearching": false,
+        "Sniff": true
+    },
     "TeamSettings": {
         "SiteName": "Mattermost",
         "MaxUsersPerTeam": 50,