Define `public/assets/` folder to be the base for images; defined in the project config

[andrewrubin]

Bringing this over from an open ticket we were tracking elsewhere.

The idea:

• User-defined assets directory in the public folder; set up in the corgi project config

The goal:
WTC projects would often have a phase where the localized builds would consist of combinations of "full" and "teaser" phases — for example, English would be the full project, while the localized builds would be "teasers". These "teaser" builds would live in a separate repo, and have to be compiled separately, and then dropped into the main ("full") build. This would always introduce conflicting asset paths. With this proposed setup, you could just tweak the config in the teaser builds to pull from teaser-assets instead of assets.