Add package manager graphs to /about/stats

[mattbk]

This would help show growth in each of the package managers (now only npm) over time without having to run database queries.

Could also do this if #3938 included returning the number of packages during search.

[mattbk]

Also add as an API endpoint. That may be easier to start with.

[mattbk]

Partly covered at https://gratipay.com/on/npm/ now:

[mattbk]

Partly covered

Should also include the total number of packages so we can do %. This number is different today:

224 out of all 500,358 npm packages are on Gratipay.