Fix ancestry on newly created units

[wilson29thid]

Units created after the introduction of the units.ancestry column don't have a value for that. We'll need to dynamically generate it, and then hopefully switch people to v3 for creating/modifying units.

[wilson29thid]

Updated manually using:

UPDATE `personnel_v2`.`units`
SET ancestry = trim(both '/' from path)
WHERE (`path` <> '/') 
    AND (`ancestry` IS NULL)

And deprecation notice added to units page in v2 admin via https://github.com/29th/personnel-api/commit/6a44a6e1e3a201bda4918a21ad159cf262cd9163