FIX: Email Send post has already been taken error (#16992)

* FIX: Email Send post has already been taken error

Adding a failing test first before coming up with a good solution.

Related: 357011eb3b4e9c5fb860a34ecbb2f5cb89e89a7a

The above commit changed

```
PostReplyKey.find_or_create_by_safe!
```

to

```
PostReplyKey.create_or_find_by!
```

But I don't think it is working as a 1-1 replacement because of the
`Validation failed: Post has already been taken` error we are receiving
with this change. Also we need to make sure we don't re-introduce any
concurrency issues.

Reported: https://meta.discourse.org/t/224706/13

* Remove rails unique constraint and rely on db index

I believe this is what is causing `create_or_find_by!` to fail. Because
we have a unique constraint in the db I think we can remove this rails
unique constraint?

* clean up spec wording

spec/lib/email/sender_spec.rb

@@ -767,6 +767,14 @@ describe Email::Sender do
           expect(post_reply_key.post_id).to eq(post.id)
           expect { email_sender.send }.to change { PostReplyKey.count }.by(0)
         end
+
+        it 'should find existing key' do
+          existing_post_reply_key = PostReplyKey.create(post_id: post.id, user_id: user.id)
+          expect { email_sender.send }.to change { PostReplyKey.count }.by(0)
+          post_reply_key = PostReplyKey.last
+          expect(post_reply_key).to eq(existing_post_reply_key)
+        end
+
       end
     end
   end