Improved mentions, now works in IE, Firefox and Chrome

Improved mentions, now works in IE, Firefox and Chrome

 * _getDropdownMenuPosition() now uses the browser's getBoundingClientRect() to get the precise offsets and height
 * _setUsername() now entirely relies on native DOM functions, this prevents issues with Firefox and Internet Explorer. As a bonus, the editor is now able to properly undo all changes
 * moved _setUsername() since it shares a lot of code with the function above, makes it far easier to apply changes to both functions without massive scrolling