make possible to add <br/> in Rich Text Editor

Add support for shift+enter to add a <br/> in the middle of text to allow for more control on spacing and line breaks.

Currently, if you hit enter in Builder RTE it adds <p><br/></p> which creates a lot of extra space. It is apparently not uncommon for RTE to allow shift+enter to add just a <br> in the middle of any content. I know Builder uses React-Quill for RTE so it might be useful to extend it with this feature