XYZ-87: GlobalRelay DM Exports do not Include Channel Name (#8275)

* Adding ChannelMemberHistory table records when DM channels are created. This ensures that both participants in a DM are shown in compliance export, even if only one of them typed anything

* Direct/Group Message channels now get pretty display names for the purpose of compliance exports

* Fixed string formatting in t.Fatal calls

* Changed uses of ChannelMemberHistory over to ChannelMemberHistoryResult in tests. This should have been done as a part of the work in XYZ-110, but seems to have been missed in this branch for some reason

store/sqlstore/compliance_store.go

@@ -223,7 +223,11 @@ func (s SqlComplianceStore) MessageExport(after int64, limit int) store.StoreCha
 				Posts.Type AS PostType,
 				Posts.FileIds AS PostFileIds,
 				Channels.Id AS ChannelId,
-				Channels.DisplayName AS ChannelDisplayName,
+				CASE 
+					WHEN Channels.Type = 'D' THEN 'Direct Message'
+					WHEN Channels.Type = 'G' THEN 'Group Message'
+					ELSE Channels.DisplayName
+				END AS ChannelDisplayName,
 				Users.Id AS UserId,
 				Users.Email AS UserEmail,
 				Users.Username