Add role attribute to the extended valid elements config for all elements

[odonnella2]

What is it?
Role is a global attribute with a focus on providing better web accessibility.

Why?
We have a number of clients focus on maintaining a certain WCAG compliance level. Role attributes will assist in defining elements and relationship on elements entered into the WYSIWYG.

Tech Notes:

Currently Umbraco uses tinymce's extended_valid_elements to enforce specific attributes which override any application specific configurations made. Recommendation is to add role to all elements.

  //These are absolutely required in order for the macros to render inline
  //we put these as extended elements because they get merged on top of the normal allowed elements by tiny mce
  var extendedValidElements = "@[id|class|style],-div[id|dir|class|align|style],ins[datetime|cite],-ul[class|style],-li[class|style],-h1[id|dir|class|align|style],-h2[id|dir|class|align|style],-h3[id|dir|class|align|style],-h4[id|dir|class|align|style],-h5[id|dir|class|align|style],-h6[id|style|dir|class|align],span[id|class|style|lang],figure,figcaption";

This request is similar to #10505

[iOvergaard]

This makes a lot of sense. I will mark this feature as up-for-grabs so please feel free to contribute with the proposed change!