Move media library from static/ to assets/

[gcushen]

Workaround Hugo's image processing limitation in static/.

To support #1988 in case where an image is used in multiple pages and requires processing.

Otherwise, for single use images, they can be placed in a page's own folder for processing.

• Update lib. path in CMS
• Update all code referring to media_dir (which represents the static/ location)
  • widget page BGs
  • ...
• Update staticref shortcode to enable loading PDF from assets/media/
  • DESCOPED - consider if there's need to intro similar shortcode relative to assets/ in future

Breaking changes:

Gallery is now intended only for local images in the page bundle (so that they can be optimized by Hugo).

[Shatur]

Gallery is now intended only for local images in the page bundle (so that they can be optimized by Hugo).

@gcushen, is it possible to share images between pages? I have the same image for different languages ​​(these pages have different folders).
I tried the alternative method from the documentation but it doesn't work.