Implement a changelog to further the debiting of wallet balances

Summary:
This differential maintains a changelog entry for every modification to an entitlement, as it happens.

After a period of time (most notably, 13 days, 14 days, 15 days, 1 month, 2 months, etc.), the expected charges can be calculated from that changelog.

There's a number of TODOs left;

• Pro rata temporis calculation
• Actual charging of the wallet.
• Prediction of the next charge (when, what and how much)
• Collate the 'creating' and 'charging' queries to ->sum() the values of the columns.
• Evaluate the number of duplicate 'cost' columns
• subscription period for the sku (i.e. domain registered with us -> yearly)

Test Plan:

$ vendor/bin/phpunit tests/Feature/BillingTest.php

Reviewers: #hkccp_developers, machniak

Reviewed By: #hkccp_developers, machniak

Subscribers: machniak, #hkccp_developers

Differential Revision: https://git.kolab.org/D964