Add a "comment" field in user account profile

Description

In the Administration Area, there is no way to add information to user accounts, especially for users that are external from the organization (with non LDAP account). Information such as the name of a local referee and an account expiration date would be useful.

Proposal

Add a comment field to the user account descriptor. This field should be edited by a GitLab administrator only. This would help GitLab administrators to maintain local accounts.