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/db/post_migrate/20221117052348_truncate_chat_messages_over_max_length.rb

@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+class TruncateChatMessagesOverMaxLength < ActiveRecord::Migration[7.0]
+  def up
+    if table_exists?(:chat_messages)
+      # 6000 is the default of the chat_maximum_message_length
+      # site setting, its safe to do this because this will be
+      # run the first time the setting is introduced.
+      execute <<~SQL
+        UPDATE chat_messages
+        SET message = LEFT(message, 6000), cooked_version = NULL
+        WHERE LENGTH(message) > 6000
+      SQL
+    end
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end