Convert scenario outlines feature from legacy

Also take the opportunity to add better documentation to the feature
preamble (mostly lifted from the wiki)
Move showing differences to new feature folder
Removed redundant feature

I'm not convinced this feature is really necessary. It's not clearly
documenting anything, and the test itself is pretty useless.
Moved language help to new features

Took the opportunity to make the preamble clearer.
Moved feature for snippets to new structure