Code cleanup

Quick summary

I refactored the NasaBot to improve code quality, readability, and maintainability by reducing duplicated code and separating common functionality into reusable functions.

Changes made

• Configured a reusable Axios client for the NASA APOD API using a base url, api key, and request timeout
• Created a fetchApod() function to retrieve APOD, removing duplicated API request code
• Added formatExplanation() helper function to truncate long APOD descriptions to display in slack
• Added a sendAPOD() function to format and send APOD responces, without repeated code
• Better logging

Overall, after this rework, it should be much easier to add future features and maintain the bot.