[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>

model/reaction.go

@@ -16,6 +16,7 @@ type Reaction struct {
 	UpdateAt  int64   `json:"update_at"`
 	DeleteAt  int64   `json:"delete_at"`
 	RemoteId  *string `json:"remote_id"`
+	ChannelId string  `json:"channel_id"`
 }
 
 func (o *Reaction) IsValid() *AppError {