[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/post_store_test.go

@@ -10,5 +10,5 @@ import (
 )
 
 func TestPostStore(t *testing.T) {
-	StoreTest(t, storetest.TestPostStore)
+	StoreTestWithSqlSupplier(t, storetest.TestPostStore)
 }