Sort and deduplicate username blacklist

Also convert to a plaintext newline delimited file -- there's nothing
gained from using YAML here at the moment.
Validate at registration that username is not blacklisted

This adds a validation rule in the registration schema that prevents
users from registering with blacklisted usernames.
Lazy-load username blacklist

By lazy loading the blacklist applications can include the h package
and subsequently use the `config.override_assets` API to change the
blacklist file before the first request.
Merge pull request #1571 from hypothesis/username-blacklist

Username blacklist