Rename API endpoint field for auth key generation

[josecelano]

Rename the field valid_until in the auth key generation endpoint:

POST /api/key/:expiry_time

{
  "key": "IaWDneuFNZi8IB4MPA3qW1CD0M30EZSM",
  "valid_until": "24525324523452"
  "expiry_time": "2015-05-22T14:56:28.000Z"
}

• valid_until is the current field. Timestamp.
• expiry_time is the new field. Datetime ISO with timezone.

Originally posted by @da2ce7 in torrust/torrust-index#78 (comment)

NOTES:

• Keep the current field until the next major release.

[josecelano]

Reopen this issue on the backend when this issue is finished.