Slugs in request URLs must be lowercase

The GOV.UK slugs/URLs will be lowercase, so let's enforce it to avoid a
situation where they send it uppercase but we neglect to downcase it.

We will still need to remember to downcase Section IDs in breadcrumbs and
child_section_groups before we make them into slugs as part of base_paths.
Ensure that base_paths are always lowercase

GOV.UK URLs are case sensitive and the styleguide says they must be lowercase:
https://insidegovuk.blog.gov.uk/url-standards-for-gov-uk/
Add rules to README
Slugs in specs should conform to the styleguide

Practice what you preach.