fire update

enabled fire to destroy floors and objects
also tweaked the behaviour of glass to enable glasslocks against fire
also made fire extingiushers stop being useless
Update power_cells.dm

derped a debug message into the commit
Update changelog.html

underped the changelog too
Update mineral.dm

removed sheettpye = "plastic" again because it snuck back in after being removed already
Merge pull request #3316 from LordBraindead/bleeding-edge-freeze

fire update