Add new keg sizes

[mik3y]

From the forum:

Can you please tell me how to edit the keg size dropdown list when adding a keg?

I need to add 3 additional sizes:

1) European 30L kegs (different to the US 7.75 gallon) 
2) 2.5 gallon Corney
3) 3 gallon Corney.

A lot of the import beer I buy from the UK is only available in the 30 liter size and my friends and I use both the 2.5 and 3 gallon corney's for wine.

See also #317