Make the sync polling interval configurable

[garethbowen]

The sync polling interval is hardcoded to 10 minutes but for our use case and on our RapidPro server we'd like a much shorter interval, for example, 1 minute. Please make this variable configurable through settings rather than hardcoded.

[nicpottier]

Syncing really should only be a last resort, Firebase should be waking your device to cause it to sync automatically if you have everything correctly configured. But if you want to submit a PR to make this an advanced setting then we can take a look.

[Hareet]

Hi @nicpottier !

I've created a Firebase console config and placed the API key in temba/settings.py as per your comment here: #20

There was some configuration related to google-services.json and inserting code into the application from Google Firebase docs. Do you have any further docs on RapidPro's side or pointers to verify the configuration is working as intended?