this commit denormalises the Tags related to a Pre auth key
back onto the preauthkey table and struct as a string list.
There was not really any real normalisation here as we just added
a bunch of duplicate tags with new IDs and preauthkeyIDs, lots of
GORM cermony but no actual advantage.
This work is the start to fixup tags which currently are not working
as they should.
Updates #1369
Signed-off-by: Kristoffer Dalby <kristoffer@tailscale.com>
