[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

store/sqlstore/user_store.go

@@ -67,15 +67,11 @@ func NewSqlUserStore(sqlStore SqlStore, metrics einterfaces.MetricsInterface) st
 		metrics:  metrics,
 	}
 
-	us.usersQuery = sq.
+	us.usersQuery = us.getQueryBuilder().
 		Select("u.*", "b.UserId IS NOT NULL AS IsBot").
 		From("Users u").
 		LeftJoin("Bots b ON ( b.UserId = u.Id )")
 
-	if us.DriverName() == model.DATABASE_DRIVER_POSTGRES {
-		us.usersQuery = us.usersQuery.PlaceholderFormat(sq.Dollar)
-	}
-
 	for _, db := range sqlStore.GetAllConns() {
 		table := db.AddTableWithName(model.User{}, "Users").SetKeys(false, "Id")
 		table.ColMap("Id").SetMaxSize(26)