[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>

db/migrations/postgres/000089_add-channelid-to-reaction.down.sql

@@ -0,0 +1 @@
+ALTER TABLE reactions DROP COLUMN IF EXISTS channelid;

db/migrations/postgres/000089_add-channelid-to-reaction.up.sql

@@ -0,0 +1,3 @@
+ALTER TABLE reactions ADD COLUMN IF NOT EXISTS channelid varchar(26) NOT NULL DEFAULT '';
+UPDATE reactions SET channelid = (select channelid from posts where posts.id = reactions.postid) WHERE channelid='';
+CREATE INDEX IF NOT EXISTS idx_reactions_channel_id on reactions (channelid);