Allow medics to 'mark' downed teammates.

Often, people assume that they aren't going to be revived and give up, especially if the nearby medics aren't in their fireteam/squad.

A feature that would let medics 'mark' downed teammates would let them know, either by a message in the UI or narrator voice line, that they are going to be revived.

This would reduce the amount of players respawning before a medic can get to them, and would also let them prepare themselves for action in case there is a gunfight going on nearby.