This pull request introduces a series of improvements to Plausible.Billing.Plans, including:
* Tag the JSON file with the plan version
* Rename the JSON field limit to monthly_pageview_limit
* Move site_limit function to Billing.Plans
* Refactor subscription_interval, allowance and site_limit functions
* Remove unused AnalyzePlans task
This commit introduces a series of improvements on the Plans module including function renaming, documentation and readability. This is the groundwork for billing plans.
There should be no actual changes with this commit, therefore no changes in tests either.
