Add support for trailing slashes

Our site can expose the same page with or without a trailing slash. e.g.:
`https://site.com/content`
and
`https://site.com/content/`

The `prefixes` parameter in `@DeepLinkSpec` has been immensely helpful for reducing duplicated paths, but we still have to duplicate each one with a trailing slash. It would be nice if they could be handled by DeepLinkDispatch.

Start by tracing how the @DeepLinkSpec prefixes parameter is processed. The change is complete when DeepLinkDispatch handles both a path with and without a trailing slash, so users do not need to duplicate each path.

Written by the indexing model from the issue text.