[MM-55143] Disallow reacting with an emoji that does not exist, limit the total number of unique reactions per post (#25331)

* [MM-55143] Disallow reacting with an emoji that does not exist

* WIP for server limit on emoji reactions

* WIP

* Implement default limit of 25 unique emoji reactions

* Add modal for reaction limit

* Fix test

* PR feedback

* Fix i18n

* Update admin string

* Merge'd

* Fixing some issues, check limits correctly based on other users reactions

* Fix typos

* Fix lint/test

* Add tests, fix other tests

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Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/store/store.go

@@ -744,6 +744,8 @@ type ReactionStore interface {
 	Delete(reaction *model.Reaction) (*model.Reaction, error)
 	GetForPost(postID string, allowFromCache bool) ([]*model.Reaction, error)
 	GetForPostSince(postId string, since int64, excludeRemoteId string, inclDeleted bool) ([]*model.Reaction, error)
+	GetUniqueCountForPost(postId string) (int, error)
+	ExistsOnPost(postId string, emojiName string) (bool, error)
 	DeleteAllWithEmojiName(emojiName string) error
 	BulkGetForPosts(postIds []string) ([]*model.Reaction, error)
 	DeleteOrphanedRowsByIds(r *model.RetentionIdsForDeletion) error