refactor & test homepage alerts

[thecristen]

Scope

Asana Ticket: 📈 Speed up alerts call on the homepage

Implementation

Showing alerts on the homepage means we have to fetch all alerts, group them by applicable mode and accessibility effect, and fetch all the relating routes and stops so we could render the right route pills or stop links. When starting from an empty cache it's a lot of data fetching!

This PR does a couple things:

1. The call to fetch all alerts was doing a little bit more than it needed to -- notably, Alerts.Repo.all/1 calls a function that calls Alerts.Sort.sort/1. For the homepage, we don't need any alert sorting - so I added a function that fetches all current alerts, without the sorting.
2. Extracted the two data pipelines into two functions that I can add tests for. And rewrote them! The most impactful change there was probably using Task.async_stream/3 to manage fetching many routes or stops in parallel. Another improvement over the original code is that it doesn't break if the Routes.Repo and Stops.Repo calls return nil (which is a valid case).

How to test

I used Util.log_duration/3 to measure how long it takes to render the alerts tab HTML content given a list of alerts. All numbers are in ms

• this branch, no cache - 883.581
• this, everything cached - 3.682
• main branch, no cache - 8338.736
• main, everything cached - 6.147