New mentions identification

A user must only be notified on the first save after the introduction of a mention. That does not prevent the user to be notified several times (eventually for a single document save) if her name is newly introduced in several places of a text at once.

See the Implementation for more details of the realization of this feature.

See https://design.xwiki.org/xwiki/bin/view/Proposal/UserMentions#HUC5.Mentioninthesyntax