Request AI RTB

[martinco]

Originally reported by: Martin Collins

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• group leaders action item
• request rtb
• only when not within active AO
• marks lz for group members
• flys in and drops smoke prior to arrival
• once inside, group leader has action to liftoff
• invulnerable to get home safe
• lands at base
• ejects all
• flies off and despawn

[martinco]

Original comment by Jon Hawkes:

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Linked to #197 (signage to accompany new functionality)