Make the CO2 estimate units clear

[samsucik]

I noticed that, currently, Carbon bot reports the CO2 estimates in tons but it's completely unclear to the end user that the bot really means short tons -- the US version of a ton, as opposed to long tons used, for instance, in the UK. And a European user might be confused altogether because they know tonnes which are different still and not used by the bot.

I think we need to make it very clear what units the bot uses. The simplest way around it might actually be giving the estimates in kilograms as those are much more universal.