Create scheduled function that purges old data

[johnnyoshika]

Delete these documents via Firebase Scheduled Function:

• idtoken -> after 24 hours
• contexttoken -> after 24 hours
• accesstoken -> after 1 hour
• nonce -> after 1 minute (LTIJS' MongoDB provider purges in 10 seconds, but we'll allow more time in case of network latency)
• state -> after 10 minutes

[johnnyoshika]

Scheduled function created here: https://github.com/examind-ai/ltijs-firestore-scheduler