Add income and spending insight cards to dashboard

[JoshAntBrown]

This PR implements the income and spending insight cards for the dashboard.

Requirements

• Should generate a time series of income and spending for a Family, scoped to Period.last_30_days
  • Each data point in the series represents a "Trailing 30 day sum" of income (negative transaction values)
  • The "headline" number shows the last value in the series
  • The "trend" value is the overall time series trend
• Should find the top 3 accounts with the most income or spending within the last 30 days and display the sum
  • Each account is an anchor that goes to that account page
  • Bank icons are not in-scope (just use a generic letter as shown in designs)
  • A "more" counter should show if there are more than 3 accounts
• Display a trendline of series. No tooltips, no axes.
• These insights should be included in the dashboard

/claim #645
Resolves #645

[JoshAntBrown]

/claim #645

[zachgoll]

The structure of this looks good to me, just added some notes regarding tests / exchange rate conversions for handling multi-currency.

[JoshAntBrown]

I've had a bit of a refactor and added a test in a similar fashion to the other account snapshot.

Hopefully this is a bit more manageable now without straying too far from Active Record conventions.

Here's my copy of the test cases, updated to include spending and income on a rolling basis:
https://docs.google.com/spreadsheets/d/112tRiy0l8WWngEGMdEX2JprYJmYWUW--B2UDUiW9nOE/edit

Here's how it looks with the demo data:

[zachgoll]

Looks great, thanks for adding those test cases! I have copy/pasted the expected results into that main spreadsheet for now and we'll eventually get all those into an easier format to collaborate on.

[Samuelodan]

Ah, that's an interesting solution. I was starting to get a hang of the models, the associations between them, and the TimeSeries, but even if I got a few more days, I doubt I would've completed the feature.

I guess I'll hang around in this repo and slowly learn how the whole thing is put together.

You all are doing amazing work.
Thanks for the opportunity to contribute. ✨