Show API token times in the admin's timezone

[rbruhn]

Fixes the expiry time on the API Tokens page (#569).

The problem

The expiry picker read the time you picked as UTC, the app's timezone. On development, picking 2:15 PM New York time at 2:00 PM was read as 14:15 UTC, four hours in the past, and was refused with "must be a date after now". A time that was accepted was saved four hours off, and the list showed every time in UTC.

The fix

• The Expires at picker uses the signed-in admin's profile timezone (falling back to the app's UTC when there is no profile), and its help text names that timezone.
• The after now rule is replaced by a minimum of now in that timezone, so the browser's picker also blocks earlier times.
• The Last used, Expires and Created columns are shown in the same timezone.
• Times are still stored in UTC.

Tested locally

• tests/Feature/Admin/ApiTokenResourceTest.php: 9 passed. New cases, with the clock at 18:00 UTC and an admin in America/New_York: 3:00 PM is accepted and saved as 19:00 UTC; 1:00 PM is refused; a token expiring at 19:00 UTC is listed as 15:00.

To verify on development

See #567.