Move implant weight to surgery model

[x] Add implant_weight field to Surgery model (+ve float, non-nullable, blank).
[x] Modify water control model to subtract last surgery implant weight from each weighing.
[x] Modify water restriction admin view to show non-editable implant weights.
[x] Add migration to remove implant weight from Subject model; copy implant weight to subject JSON field.
[x] Run script to move implant weight values to first headplate implant surgery or to first surgery.
[x] Deploy to cortexlab Alyx dev
[x] Deploy to production

cortex-lab / alyx