Update justdelete.me to work in directories

While `justdelete.me` runs on a self-hosted, single-serving site at http://justdelete.me/ it could also be hosted locally on a devs machine, where it *may* not be running on the root of that host.

This pull request fixes the links on the site to use `relative` linking instead of `absolute` (especially with stylesheets) so a dev can run justdelete.me and test it just about anywhere they choose.

... this still maintains the ability to *work* correctly as if it was running from root (thanks relative links), but clearly improves a dev's ability to just drop it on an already existing web server for testing, with minimal fuss and impact.

Based on my testing, this should have zero changes seen by the end user, and works fine running as root (/var/www/) or else (/var/www/justdelete.me) for example.

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... I also took the time to update the `contrib` php and json documents.