[MM-45444] Denormalize Reactions to add ChannelId for top reactions insights query (#20572)

* Denormalize Reactions to add ChannelId for top reactions insights query

* Remove hardcoded timestamps

* Fix store, api4 tests for reactions

* Fix tests

* Fix integrity tests, allow reaction to have ChannelId populated before calling store function

* Lint fixes

* Add ChannelId field to BulkGetForPosts, Delete store handlers

* Add index to mysql migration, add not null characteristic without a separate command

* Select channelId instead of fetching post via store.GetPost, add if exists to drop column

* Make updating of Reactions conditional to support pre-migration

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

store/sqlstore/integrity_test.go

@@ -212,6 +212,7 @@ func createReaction(ss store.Store, userId, postId string) *model.Reaction {
 		UserId:    userId,
 		PostId:    postId,
 		EmojiName: model.NewId(),
+		ChannelId: model.NewId(),
 	}
 	reaction, _ = ss.Reaction().Save(reaction)
 	return reaction