Enable VisualEditor for NS_FILE, NS_HELP, NS_CATEGORY

VisualEditor should work fine in these three namespaces, as they are not
signed and do not have fragments of partial wikitext.

Also clean up MWwiki's VE namespace definition to extend, not replace.

Bug: 55968
Change-Id: I2d378203b4459fd459653a18ca4a060c43086a65