The platform_img() helper is missing its front-end code

[alexgibson]

Description

We have a platform_img() helper in bedrock [1], however it's not been used on any pages in a number of years, and it looks like the front-end code no longer exists to support it either (my guess it was removed when we migrated away from jQuery).

We could resurrect (and modernize) the front-end JS to support it, but it's probably only worth investing time in if we think the helper might still be useful today. Given that responsive images are now quite a bit more complicated (supporting things like srcset and sizes), it may require a rethink on the approach for how platform_img() works entirely.

Interested in thoughts from others here, and if they think such a helper is still worth investing time in to fix.

[1] https://bedrock.readthedocs.io/en/latest/coding.html#platform-img