feat(v2): Sort disruptions by start or end date

[jzimbel-mbta]

Summary of changes

Asana Ticket: 🏹 Implement sort by date functionality

Based on the ticket's acceptance criteria, I needed to maintain the state of each sortable column. Otherwise, one of these behaviors would be necessary:

• toggling the sort direction for one column toggles it for all of them, or
• changing the active sorted column resets the previous sorted column to a default
  (this is what the v1 disruptions table does)

The new sorting filters look like:

%{
  ...
  sort: %{field_name => :asc | :desc},
  active_sort: field_name
}

which allows us to remember the sort direction of all columns, even when they are not active.

Reviewer Checklist

• Meets ticket's acceptance criteria
• Any new or changed functions have typespecs
• Tests were added for any new functionality (don't just rely on Codecov)
• This branch was deployed to the staging environment and is currently running with no unexpected increase in warnings, and no errors or crashes.

[jzimbel-mbta]

Still finishing up tests!