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.
Martin Brennan
Roman Rizzi