[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/i18n/en.json

@@ -6526,6 +6526,10 @@
     "id": "app.reaction.save.save.app_error",
     "translation": "Unable to save reaction."
   },
+  {
+    "id": "app.reaction.save.save.too_many_reactions",
+    "translation": "Reaction limit has been reached for this post."
+  },
   {
     "id": "app.recover.delete.app_error",
     "translation": "Unable to delete token."