* Modify backend to allow expiration of API Keys

* Add middleware test for expired api keys

* Modify frontend to enable expiration of API Keys

* Fix frontend tests

* Fix migration and add index for `expires` field

* Add api key tests for database access

* Substitude time.Now() by a mock for test usage

* Front-end modifications

* Change input label to `Time to live`
* Change input behavior to comply with the other similar
* Add tooltip

* Modify AddApiKey api call response

Expiration should be *time.Time instead of string

* Present expiration date in the selected timezone

* Use kbn for transforming intervals to seconds

* Use `assert` library for tests

* Frontend fixes

Add checks for empty/undefined/null values

* Change expires column from datetime to integer

* Restrict api key duration input

It should be interval not number

* AddApiKey must complain if SecondsToLive is negative

* Declare ErrInvalidApiKeyExpiration

* Move configuration to auth section

* Update docs

* Eliminate alias for models in modified files

* Omit expiration from api response if empty

* Eliminate Goconvey from test file

* Fix test

Do not sleep, use mocked timeNow() instead

* Remove index for expires from api_key table

The index should be anyway on both org_id and expires fields.
However this commit eliminates completely the index for now
since not many rows are expected to be in this table.

* Use getTimeZone function

* Minor change in api key listing

The frontend should display a message instead of empty string
if the key does not expire.