Fix #326: Check for uptake goals

[leplatrem]

The goal of this check is to measure the percentage of clients reached after a certain amount of time.

[checks.remotesettings-uptake.goals]
description = ""
module = "checks.remotesettings.uptake_goals"
params.server = "https://firefox.settings.services.mozilla.com/v1"
params.channels = ["nightly"]
params.total_clients = 100000
params.goals.1200 = 5
params.goals.3600 = 50
params.goals.7200 = 90