[Rich Content Editor] Allow responsive images -- don't remove srcset or sizes attribute from img tag

[joe_spencer]

Currently the rich content editor strips the srcset and sizes attribute from the img tag. These features have established cross browser support and make it easier to include responsive images.

[natasha_sutevsk]

In the new RCE users can specify pixel dimensions for an image (or size presets), however, this means that images are not responsive across devices which negatively impacts the student experience. Having responsive elements is an essential feature for mobile first development and important given the range of devices that students have. Without this feature, it also means we need to guide instructors through the html to create a relative (%) width for images.

It would be great if, in the image options (custom size), the field for pixel width allowed entering a percentage width. Alternatively, perhaps in addition to small, large, custom, etc there was a 'relative' or 'responsive' option that could accept % widths and/or heights.

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@eschiebel