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
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