Add feature to allow LDAP/AD users avatar image to be used in preference to Gravatar

Created by: thechile

Allow a user’s LDAP/AD stored avatar thumbnail to be queried and if found, used in preference to the Gravatar service.

This feature was also referenced in:

• http://feedback.gitlab.com/forums/176466-general/suggestions/5158352-use-photo-thumbnail-via-ldap-vs-gravatar
• https://github.com/gitlabhq/gitlabhq/issues/3107