Introduce a stimulus controller for bubbles

The "closing soon" bubbles were introduced in #406, and the "falling
back" bubbles in #500. However, these bubbles are part of the cached
card and so as time passes, the relative time doesn't change unless
the card is touched.

This PR introduces a stimulus controller for bubbles, which takes care
of:

- making the bubble visible during the reminder period
- calculating the relative number of days until entropy kicks in
- rendering appropriate text around the day count

ref: https://37s.fizzy.37signals.com/collections/693169850/cards/999009091

app/javascript/helpers/date_helpers.js

@@ -2,6 +2,10 @@ export function differenceInDays(fromDate, toDate) {
   return Math.round(Math.abs((beginningOfDay(toDate) - beginningOfDay(fromDate)) / (1000 * 60 * 60 * 24)))
 }
 
+export function signedDifferenceInDays(fromDate, toDate) {
+  return Math.round((beginningOfDay(toDate) - beginningOfDay(fromDate)) / (1000 * 60 * 60 * 24))
+}
+
 export function beginningOfDay(date) {
   return new Date(date.getFullYear(), date.getMonth(), date.getDate())
 }