MM-11781: Basic Data Export Command Line. (#9296)

* MM-11781: Basic Data Export Command Line.

* ChannelStore new unit tests.

* TeamStore new unit tests.

* Unit test for new UserStore function.

* Unit tests for post store new methods.

* Review fixes.

* Fix duplicate command name.

i18n/en.json

@@ -179,6 +179,14 @@
     "id": "api.channel.leave.last_member.app_error",
     "translation": "You're the only member left, try removing the Private Channel instead of leaving."
   },
+  {
+    "id": "app.export.export_write_line.json_marshall.error",
+    "translation": "An error occured marshalling the JSON data for export."
+  },
+  {
+    "id": "app.export.export_write_line.io_writer.error",
+    "translation": "An error occurred writing the export data."
+  },
   {
     "id": "api.channel.leave.left",
     "translation": "%v left the channel."