Fix handling of Vendor CSS in build and compile tasks.

Prior to this commit, for both `build` and `compile` tasks, `vendor_files.css` was separately linked as resources in the index, but the files were never copied to `build` or `bin`, however they were concatenated into the app's CSS.

Handling is now consistent with JS handling for these respective tasks, i.e.:
- build: `vendor_files.css` are copied to `build` and separately linked in `index.html`.
- compile: `vendor_files.css` are concatenated and recess'd with the app's CSS and not linked in `index.html`.