New API methods to retrieve last activity date
### Description
Last activity date for a user field in the DB creates problems (see https://gitlab.com/gitlab-org/gitlab-ee/issues/1158). We need to move the data to Redis and query it with an API.
### Proposal
* Create new methods in the API to retrieve last activity date
* Restricted to admin role
* Retrieve the information:
* User information
* Last activity timestamp
* Ordered by oldest by default
cc @jameslopez
issue