DEV: Replace instances of Discourse.base_uri with Discourse.base_path
This is clearer because the base_uri is actually just a path prefix. This continues the work started in 555f467.
You can now add javascript files under `/javascripts/*` in a theme, and they will be loaded as if they were included in core, or a plugin. If you give something the same name as a core/plugin file, it will be overridden. Support file extensions are `.js.es6`, `.hbs` and `.raw.hbs`.