DEV: correct a relationship – a chat message may have several mentions (#20219)

This change only makes the model reflect correctly what's 
already happening in the database. Note that there are no calls 
to chat_message.chat_mention in Core and plugins so this 
change should be safe.

Also note, that at the moment we use the chat_mentions db 
table only to support notifications about mentions, but 
we're going to start using it for other cases. This commit is 
the first step in that direction.

plugins/chat/spec/models/chat_message_spec.rb

@@ -5,6 +5,8 @@ require "rails_helper"
 describe ChatMessage do
   fab!(:message) { Fabricate(:chat_message, message: "hey friend, what's up?!") }
 
+  it { is_expected.to have_many(:chat_mentions).dependent(:destroy) }
+
   describe ".cook" do
     it "does not support HTML tags" do
       cooked = ChatMessage.cook("<h1>test</h1>")