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