Default priority should be normal, not high

[c4talyst]

Current behaviour

• low_priority=True: FCM is called with normal priority
• low_prority=False: FCM is called with high priority
• The default is high priority

Expected behaviour

• Priority should default to normal
• The client should be able to adjust to high priority
• low priority doesn't exist on the FCM API

Rationale

• High priority has quotas to prevent abuse (QuotaExceededError: Topic quota exceeded.) & (requests.exceptions.HTTPError: 429 Client Error: Too Many Requests
• It's confusing that low means normal and not low means high; when you might assume that not low means normal.

Thanks for a great library :) We use it A LOT