Fixes and improvements to the final Corebot build.

• Slow slash commands show "thinking" instead of failing, and add-on commands answer slash commands properly.
• Slash commands register once at start-up, keep the same place every time, never appear twice, and up to 200 fit (100 per server, 100 global).
• Add-ons written for older Corebot versions load and run again.
• When a ticket category is full, new tickets open in an overflow category.
• The temp channel and command channel buttons work again, and level roles no longer cause errors.
• Every command loads before the bot reports them, so fresh installs record all of them.
• Unloading or reloading an add-on removes its listeners on Linux hosts too.
• At start-up the bot lists how many commands are on and which ones are turned off.

To update, replace the src folder, index.js and package.json. Your settings, data and add-ons stay as they are.