[MM 12464] Include DM/GM Channels and Their Posts in the Bulk Export (#10421)

* transplant the existing PR into the working tree

* start addressing review comments

* move existing direct channel export code into this branch

* modify channel exporter to use squirell and populate members in two steps

* use squirrel to build sql queries for channel and dm/gm export methods

* remove debug helpers and use Username instead of UserId

* unit test for DM Channel exporter

* add more unit tests for channel export

* add test for DM/GM post export

* checkpoint with failing test for postgres

* use getQueryBuilder to make sure squirrel uses the correct formatting
for each database

* add a test for post export

* fix shadowed vars that broke the build

* address review comments and add tests to support it

* address review comments and add a mlog call

* s/Info/Debug/

* address review comments in post_store

* address review comments in channel_store

* address review comments in export

* address review comment in post_store: drop GroupBy

* address review comment on supplier: move getQueryBuilder to sqlstore

* address review comments: explicit TearDown

* address review comments: improve test coverage

* address review comments: make sure public and private channels are excluded

* address review comments: improve test coverage

* address review comments: make sure Channels table gets truncated after
each test

* more cleanups and better assertions

* wrap PostStore in a StoreTestWithSqlSupplier

* last minute changes: improve post export test coverage and check members

* address review comments: make sure all posts have their channel
members set

* address review comments: make sure all posts have their ChannelMembers
exported correctly

* gofmt fix

* sort channels so it's possible to assert on index

app/export_converters.go

@@ -38,6 +38,16 @@ func ImportLineFromChannel(channel *model.ChannelForExport) *LineImportData {
 	}
 }
 
+func ImportLineFromDirectChannel(channel *model.DirectChannelForExport) *LineImportData {
+	return &LineImportData{
+		Type: "direct_channel",
+		DirectChannel: &DirectChannelImportData{
+			Header:  &channel.Header,
+			Members: channel.Members,
+		},
+	}
+}
+
 func ImportLineFromUser(user *model.User, exportedPrefs map[string]*string) *LineImportData {
 	// Bulk Importer doesn't accept "empty string" for AuthService.
 	var authService *string
@@ -141,6 +151,18 @@ func ImportLineForPost(post *model.PostForExport) *LineImportData {
 	}
 }
 
+func ImportLineForDirectPost(post *model.DirectPostForExport) *LineImportData {
+	return &LineImportData{
+		Type: "direct_post",
+		DirectPost: &DirectPostImportData{
+			ChannelMembers: post.ChannelMembers,
+			User:           &post.User,
+			Message:        &post.Message,
+			CreateAt:       &post.CreateAt,
+		},
+	}
+}
+
 func ImportReplyFromPost(post *model.ReplyForExport) *ReplyImportData {
 	return &ReplyImportData{
 		User:     &post.Username,