FIX: Make deleted topic post bookmarks more resilient (#10619)

This PR ensures that new bookmarks cannot be created for deleted posts and topics, and also makes sure that if a bookmark was created and then the topic deleted that the show topic page does not error from trying to retrieve the bookmark reminder at.

spec/lib/bookmark_manager_spec.rb

@@ -21,6 +21,16 @@ RSpec.describe BookmarkManager do
       expect(bookmark.topic_id).to eq(post.topic_id)
     end
 
+    it "when topic is deleted it raises invalid access from guardian check" do
+      post.topic.trash!
+      expect { subject.create(post_id: post.id, name: name) }.to raise_error(Discourse::InvalidAccess)
+    end
+
+    it "when post is deleted it raises invalid access from guardian check" do
+      post.trash!
+      expect { subject.create(post_id: post.id, name: name) }.to raise_error(Discourse::InvalidAccess)
+    end
+
     it "updates the topic user bookmarked column to true if any post is bookmarked" do
       subject.create(post_id: post.id, name: name, reminder_type: reminder_type, reminder_at: reminder_at)
       tu = TopicUser.find_by(user: user)