[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

model/channel.go

@@ -73,6 +73,11 @@ type ChannelForExport struct {
 	SchemeName *string
 }
 
+type DirectChannelForExport struct {
+	Channel
+	Members *[]string
+}
+
 func (o *Channel) DeepCopy() *Channel {
 	copy := *o
 	if copy.SchemeId != nil {

model/channel_member.go

@@ -50,6 +50,7 @@ type ChannelMembers []ChannelMember
 type ChannelMemberForExport struct {
 	ChannelMember
 	ChannelName string
+	Username    string
 }
 
 func (o *ChannelMembers) ToJson() string {

model/post.go

@@ -122,6 +122,12 @@ type PostForExport struct {
 	ReplyCount  int
 }
 
+type DirectPostForExport struct {
+	Post
+	User           string
+	ChannelMembers *[]string
+}
+
 type ReplyForExport struct {
 	Post
 	Username string