Rolling retention widget

[timgl]

Changes

Previously when adding retention to the dashboard it would have a fixed date and never update. This fixes that by

• by default not setting a selectedDate as the filter
• adding an X button if you have selected a date so you can easily unselect it
• add a migration to remove all selectedDate from any retention dashboard item, as I think all of those were not meant to be set.

Checklist

• All querysets/queries filter by Organization, by Team, and by User
• Django backend tests
• Jest frontend tests
• Cypress end-to-end tests

[macobo]

This works, though failing test.

Not-directly-related-to-pr: I hadn't used retention in a while and the title Current date confused me until I played around with it. Maybe worth renaming this to e.g. Last date?

Good job!