[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

---------

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

server/channels/app/export_test.go

@@ -29,18 +29,20 @@ func TestReactionsOfPost(t *testing.T) {
 	reactionObject := model.Reaction{
 		UserId:    th.BasicUser.Id,
 		PostId:    post.Id,
-		EmojiName: "emoji",
+		EmojiName: "smile",
 		CreateAt:  model.GetMillis(),
 	}
 	reactionObjectDeleted := model.Reaction{
 		UserId:    th.BasicUser2.Id,
 		PostId:    post.Id,
-		EmojiName: "emoji",
+		EmojiName: "smile",
 		CreateAt:  model.GetMillis(),
 	}
 
-	th.App.SaveReactionForPost(th.Context, &reactionObject)
-	th.App.SaveReactionForPost(th.Context, &reactionObjectDeleted)
+	_, err := th.App.SaveReactionForPost(th.Context, &reactionObject)
+	require.Nil(t, err)
+	_, err = th.App.SaveReactionForPost(th.Context, &reactionObjectDeleted)
+	require.Nil(t, err)
 	reactionsOfPost, err := th.App.BuildPostReactions(th.Context, post.Id)
 	require.Nil(t, err)