Until recently, the only side-loaded resource was the HLJS library for
computing syntax highlighting. For this task, the gr-syntax-lib-loader
provided an interface to load the library whether or not PG is being
served from a CDN.
With this change, the component is refactored to allow loading resources
other than the syntax library. A method is added for loading the
"dark-theme" document independently of whether a CDN is configured.
Also, some documentation comments are added to the existing methods.
Change-Id: I9891539cd4cf76ac0fe430ff3988e3a9dfbb0ca3
e9bf51941a