This change updates the API tokens config page for most of the

features supported by the new API token generator:

Long API tokens are displayed nicely, inside an <input> element with

  a copy button.
A label shows when each token was last used.
Tokens which are expired or invalidated will show when that happened,

  and in the case of invalidation, will show the message.
Tokens with an expiration date will show the time left.

There's still some future work left to do in later changes:

Allow people to set the expiration date.
Show a warning when a token was created with a deprecated generator

  class.

Based on work by Michelle Aubin at /r/12492/

Ran unit tests.
Ran js-tests.
Set up a bunch of different API tokens with various expiration dates

  and validity settings. Checked that everything displayed correctly.
Checked that the copy button functioned as expected.