Minor DB refactor, in preparation for version 2.5

1.) removed the last of the PHP code columns from the database
2.) added an unknown_user field to the bots table, allowing per-bot
settings for what the
will call someone they don't know
3.) Altered the chatbot general config page in the admin to reflect the
above changes
4.) added a bot_id field and dropped a user_id field in the
unknown_inputs table
5.) dropped the user_id field from the undefined_defaults table, as it
was not used
6.) added a "memory bailout" feature to the function that returns
potential AIML categories
to prevent memory overflow errors
7.) other miscellaneous minor bug fixes