Refactor user handler to isolate whatsapp-specific logic from rest of checking mechanism

[sarge1989]

Context/Rationale

• Eventually, we might want to expand sources of new instances/messages beyond whatsapp, e.g. telegram or even API.
• Currently, the newTextInstanceHandler and newImageInstanceHandler are coupled together with whatsapp-specific logic, but with slight changes these two functions can probably be hived off.

Remarks

• both functions newTextInstanceHandler and newImageInstanceHandler should be refactored out into its own newInstanceHandler
  • newImageInstanceHandler should accept a generic download URL and token as parameters, rather than "mediaId" which is whatsapp specific
• newInstance handler should be platform agnostic. Perhaps it should be pub/sub triggered, with a new instance object being created in the queue (https://firebase.google.com/docs/functions/pubsub-events?gen=2nd)
• remaining whatsapp-specific logic could be shifted back into the webhook handler for whatsapp?

[sarge1989]

@btjl when you are free and done with the website tasks, can work on this I guess! It's not super urgent and will give you a good platform to properly get familiar with the most core parts of the codebase and architecture.

Once you are familiar with the code and what needs to be done here, let's discuss the schema of the instance data object to be put into the pub/sub queue. We can also discuss other possible architectures if needed.

[btjl]

Having looked through the code, I think not just the handlers but the instance object itself has many WhatsApp-specific properties. Curious if you'd like to keep it this way, so something like a parent instance class representing properties that are platform agnostic, followed by extending the class to platform-specific properties?

[sarge1989]

Yeah @btjl I agree! We can do something like the above. Can feel free to propose how a new data object can look like for instances. Originally it was built just for whatsapp, but I think it's worth thinking about how to make it agnostic