Remove twitter and add X icon.

[justinspencer]

Please remove the twitter icon and add the X icon. I've attached svg's for both filled and outlined icons.
I've also attached 3 versions of the X mark which are intended to go into the Symbols.

X.zip

[KateMacdonald]

@samithaf This issue was closed, however the icon change is not visible in the GEL library. The X icon should be added and the Twitter one should remain too. (see Issue#768)