This change offers several improvements to the expiration controls on the

API tokens config page:

- Users can edit the expiration of expired tokens.

- Users can set the specific time of the expiration.

- Users can set the expiration to a past date/time.
With this, the API tokens config page offers the same flexibility for editing

expiration dates as the API.
This change also resets the expired_notification_sent flag in a token's

extra_data field whenever a token's expiration is updated, so that users can

get notified again if they set a new expiration date on already expired tokens.

Ran JS unit tests.
Ran ./reviewboard/webapi/tests/test_api_token.py
Manually tested the expiration controls with various combinations of

  starting states, ending states, and with the other controls on the page.