Exponentcms Exponent CMS → 2.4.0

Replace default 'yui' interface with NEW bootstrap/jquery, leaving yui as optional

• integrate Twitter Bootstrap v4 support (it's supposedly v3 compatible, but there are some quirks)
• 'yui' would become a 'hard' framework type like bootstrap & boostrap3
• 'yui' must be specifically declared in the theme template to receive the 'yui' styling, otherwise it'll default to the NEW style
• basically all the current controls, etc...would be moved into a 'yui' subfolder and the NEW style (probably bootstrap3/4) would be moved into the root folder like controls/plugins, etc...)
• update events module
• remove ancient browser polyfill support? This would only/mostly affect forms which would still be functional. 'Ancient' browser use is strongly discouraged due to the many security failings