Add auto-update script for Chrome + Select All Categories for Sites

## Auto-Update

Follow the instructions in the `README` and this snippet will auto-update itself and any other snippets in the `FrogTrade/fos-snippets` repository.

GitHub credentials can optionally be specified in the form of `window.credentials = ['username', 'password'];` if the unauthenticated API requests hit the rate-limit.

The only tricky bit is opening Chrome DevTools on itself in order for it to work correctly, but there's an alert popup with instructions if you try to run it incorrectly.

## Select All Categories

Selects every single category and every single child category on the site tags page of the New Site Modal.