Adds elasticsearch to the user and channel autocompletion functions (#10354)

* Adds elasticsearch to the user and channel autocompletion functions

* Implement channel store GetChannelsByIds test

* Style changes and govet fixes

* Add gofmt fixes

* Extract default channel search limit to a const

* Add StringSliceDiff function to the utils package

* Honor USER_SEARCH_MAX_LIMIT on the user autocomplete api handler

* Change the elasticsearch development image

tests/test-config.json

@@ -344,9 +344,14 @@
         "Password": "changeme",
         "EnableIndexing": false,
         "EnableSearching": false,
+        "EnableAutocomplete": false,
         "Sniff": true,
         "PostIndexReplicas": 1,
         "PostIndexShards": 1,
+        "ChannelIndexReplicas": 1,
+        "ChannelIndexShards": 1,
+        "UserIndexReplicas": 1,
+        "UserIndexShards": 1,
         "AggregatePostsAfterDays": 365,
         "PostsAggregatorJobStartTime": "03:00",
         "IndexPrefix": "",