Rename "plate mail" to "plate armour".

Not sure about the name, "full plate" might sound better.

I wonder how no one else was annoyed by having three pieces of armour being
synonyms of each other.

Also, our tiles show solid plate rather than mail.

Conflicts:
	crawl-ref/source/dat/des/altar/lugonu_bribe.des
	crawl-ref/source/dat/des/portals/trove.des
	crawl-ref/source/dat/des/variable/ancient_champions.des
	crawl-ref/source/makeitem.cc