Added Check to CeloDistributionSchedule `activate` function

[soloseng]

Description

Added a check that prevents activating the CeloDistributionSchedule contract if the distributionSchedule contract address if not set in CELO token contract.

Previously, it was expected that the CeloDistributionSchedule address would be manually set in the GoldToken contract by calling setCeloTokenDistributionSchedule(). This was expected to be done before calling activate() in the CeloDistributionSchedule. However, the CeloDistributionSchedule contract did not verify that the CeloDistributionSchedule address was set in the GoldToken contract, allowing for the CeloDistributionSchedule to be incorrectly activated.

These new changes get the CeloDistributionSchedule address from the registry, instead of setting it in the GoldToken contract. Hence, no longer requiring that the CeloDistributionSchedule address be manually set in the GoldToken contract before activating the CeloDistributionSchedule contract.

Tested

unit tested

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[soloseng]

I'm not 100% sure I understand the original bug, and how this PR fixes it, so not super comfortable approving. But left some nit comments from reading through the PR.

Updated the PR description to make it more clear what the changes are and why they were needed.