Add URL slugs to the CloudPebble IDE JavaScript

This allows one to go to /ide/projects/<num>#settings to get to their project's settings. This adds  a new CloudPebble.Locations API that handles the storage of valid slugs and calls a function to activate the proper slugs on the `onhashchange` event. It also includes a function to set the current slug in the URL.

To fix a bug, the Sidebar was disavowed of it's responsibility of resource and source file switching by callback, instead stuffing a valid slug into the `a` element it generates.