Open startup

[joemasilotti]

This PR adds /open to publicly display metrics. It also adds /admin/transactions to manually add expenses and contributions.

Data is pulled from Stripe and Fathom via rake open_startup:refresh_metrics which will run once per day via bundle exec open_startup:refresh_metrics.

• Revenue is calculated from Stripe charges by summing amount.
• Expenses are calculated by adding Stripe fees (from charges) to additional fees from balance transactions. These are grouped by month and manual expenses are added in.
• Contributions are pulled from Stripe (2% of revenue to CO₂ removal) and manual entries (% of profits to women-focused communities).
• MRR is calculated by multiplying the active subscription count by $99. (This does not account for annual plans or coupons, if railsdevs ever adds either.)
• Visitors are pulled via the Fathom API thanks to @afomera's gem, fathom_api.

Pull request checklist

• Your code contains tests relevant for code you modified
• You have linted and tested the project with bin/check