Currently, to_chat is literally just a proc that does user << message.
But it'll let us do output modification in the future, especially for
something like Goon's HTML chat.
Big thanks to PJB for his to_chat script, see
https://github.com/d3athrow/vgstation13/pull/6625 for more details.
This commit allows the bartender, or anyone with access_bar, to change the
barsign's image by hitting it with an ID. This commit also adds a variable
to disable such behavior for custom maps. Finally, this commit adds two
new icons for the barsign, named simply 'on' and 'off', which look like
the base sprite for most of the other barsigns, but blank.
