[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/store.go

@@ -4,10 +4,10 @@
 package sqlstore
 
 import (
+	sq "github.com/Masterminds/squirrel"
 	_ "github.com/go-sql-driver/mysql"
 	_ "github.com/lib/pq"
 	"github.com/mattermost/gorp"
-
 	"github.com/mattermost/mattermost-server/store"
 )
 
@@ -98,4 +98,5 @@ type SqlStore interface {
 	TermsOfService() store.TermsOfServiceStore
 	UserTermsOfService() store.UserTermsOfServiceStore
 	LinkMetadata() store.LinkMetadataStore
+	getQueryBuilder() sq.StatementBuilderType
 }