Mention macro

The mention macro must allow declaring a mention. This macro has one field pointing to a single user or group.

The macro must be displayed in a distinctive and identifiable way.

Mentions to the current user and/or a group in which the current is must be highlighted in specific colors.

The text of the mention should be first name + last name, first name only or user login (the user defining the mention should be able to choose)

See https://design.xwiki.org/xwiki/bin/view/Proposal/UserMentions#HUC3.Mentionmacro