SECURITY: Limit chat message char length (#19207)

Only allow maximum of 6000 characters for chat messages when they
are created or edited. A hidden setting can control this limit,
6000 is the default.

There is also a migration here to truncate any existing messages to
6000 characters if the message is already over that and if the
chat_messages table exists. We also set cooked_version to NULL
for those messages so we can identify them for rebake.

plugins/chat/spec/requests/incoming_chat_webhooks_controller_spec.rb

@@ -19,10 +19,10 @@ RSpec.describe Chat::IncomingChatWebhooksController do
       expect(response.status).to eq(400)
     end
 
-    it "errors when the body is over WEBHOOK_MAX_MESSAGE_LENGTH characters" do
+    it "errors when the body is over chat_maximum_message_length characters" do
       post "/chat/hooks/#{webhook.key}.json",
            params: {
-             text: "$" * (Chat::IncomingChatWebhooksController::WEBHOOK_MAX_MESSAGE_LENGTH + 1),
+             text: "$" * (SiteSetting.chat_maximum_message_length + 1),
            }
       expect(response.status).to eq(400)
     end