Remove titles from breadcrumbs

We added this speculatively because we thought it might allow a better user
experience when looking at a long chain of breadcrumbs - it could be included
as the link's title.

Feedback from HMRC/their development team at iO1 is that updating a section
title near the top of the hierarchy could trigger hundreds of requests to
update every child section in the tree. The trade-off doesn't feel worth it.
Merge pull request #24 from alphagov/remove_titles_from_breadcrumbs

Remove titles from breadcrumbs
Update example section's child section descriptions

The descriptions which were initially added here were the bodies of
the child sections, which looked misleading. As far as we know, no
HMRC manual sections have descriptions so this field is optional.
If it is present, it should be a short description of the section's
content.
Merge pull request #26 from alphagov/update_section_example_description

Update example section's child section descriptions
Remove redundant .keep file
Fix test helpers for sections

Before this change, the section helper methods took parameters
but then just ignored them.
Refactor manual JSON schema validation

This introduces a reusable validator and moves JSON schema validation to the model.
Refactor section JSON schema validation
Sanitisation