Bulk import/export supports direct messages from a user to themselves (#13200)

* Test added to check support for import of messages sent to your own DM
channel

* Support for single member direct channel messages and posts

Now bulk import is able to export/import the self direct channel and
the posts inside

* Improve how the copy of channel members for DM and DM posts

* Improved the tests for the export self-channels

app/import_test.go

@@ -163,8 +163,10 @@ func TestImportBulkImport(t *testing.T) {
 {"type": "user", "user": {"username": "` + username3 + `", "email": "` + username3 + `@example.com", "teams": [{"name": "` + teamName + `", "channels": [{"name": "` + channelName + `"}], "delete_at": 123456789016}]}}
 {"type": "post", "post": {"team": "` + teamName + `", "channel": "` + channelName + `", "user": "` + username + `", "message": "Hello World", "create_at": 123456789012, "attachments":[{"path": "` + testImage + `"}]}}
 {"type": "post", "post": {"team": "` + teamName + `", "channel": "` + channelName + `", "user": "` + username3 + `", "message": "Hey Everyone!", "create_at": 123456789013, "attachments":[{"path": "` + testImage + `"}]}}
+{"type": "direct_channel", "direct_channel": {"members": ["` + username + `", "` + username + `"]}}
 {"type": "direct_channel", "direct_channel": {"members": ["` + username + `", "` + username2 + `"]}}
 {"type": "direct_channel", "direct_channel": {"members": ["` + username + `", "` + username2 + `", "` + username3 + `"]}}
+{"type": "direct_post", "direct_post": {"channel_members": ["` + username + `", "` + username + `"], "user": "` + username + `", "message": "Hello Direct Channel to myself", "create_at": 123456789014}}
 {"type": "direct_post", "direct_post": {"channel_members": ["` + username + `", "` + username2 + `"], "user": "` + username + `", "message": "Hello Direct Channel", "create_at": 123456789014}}
 {"type": "direct_post", "direct_post": {"channel_members": ["` + username + `", "` + username2 + `", "` + username3 + `"], "user": "` + username + `", "message": "Hello Group Channel", "create_at": 123456789015}}
 {"type": "emoji", "emoji": {"name": "` + emojiName + `", "image": "` + testImage + `"}}`