- Combine `sidebar/util/date.js` and `sidebar/util/time.js`. It was
   confusing to have two separate modules when there wasn't a clear
   separation of "time utilities" vs "date utilities". This also
   eliminates some redundant caching logic.

 - Rename `toFuzzyString` and `format` to `formatRelativeDate` and
   `formatDate` respectively to make the differnce between them more
   obvious

 - Add missing test for `formatDate`

 - Remove the fallback code paths for old browsers that don't support
   `Intl.DateTimeFormat`. All of our currently supported browsers do.

 - Remove unused `date` parameter from `decayingInterval` callback. The
   only consumer in `AnnotationTimestamps` didn't use it.

 - Improve types in `time.js`