increase Section slug length to 200

Resolves armstrong/armstrong#43. This changes the slug length from the default 50 to 200.

I decided not to go all the way to 255 because of `full_slug`. It's capped at 255 and is built from the slugs of each Section in the hierarchy. This allows for a bit of depth; room for a parent section or two. 200 matches with the WordPress section length making porting easier. 255 on the `full_slug` gives some padding. If the length on `full_slug` becomes a limit, that's a new Issue.