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/export.go

@@ -532,12 +532,6 @@ func (a *App) ExportAllDirectChannels(writer io.Writer) *model.AppError {
 				continue
 			}
 
-			// There's no import support for single member channels yet.
-			if len(*channel.Members) == 1 {
-				mlog.Debug("Bulk export for direct channels containing a single member is not supported.")
-				continue
-			}
-
 			channelLine := ImportLineFromDirectChannel(channel)
 			if err := a.ExportWriteLine(writer, channelLine); err != nil {
 				return err
@@ -568,12 +562,6 @@ func (a *App) ExportAllDirectPosts(writer io.Writer) *model.AppError {
 				continue
 			}
 
-			// There's no import support for single member channels yet.
-			if len(*post.ChannelMembers) == 1 {
-				mlog.Debug("Bulk export for posts containing a single member is not supported.")
-				continue
-			}
-
 			// Do the Replies.
 			replies, err := a.buildPostReplies(post.Id)
 			if err != nil {