Add maintenance mode

[psikoi]

We need a way to shut the bot down when our API is down, this can be done in two ways:

Hands off approach

• Ping https://api.wiseoldman.net/v2/ every minute or so, check if it doesn't return a 200 status code
  • If it gets more than X negative pings in a row, turn on "maintenance mode" and respond with a generic message
• No manual input from me needed, i could be asleep!

Hands on

• Read a MAINTENANCE_MODE_ENABLED env var, and respond with a generic message
• Requires me to log on to the server, update the env vars, restart the bot container