Replace Twitter with X in social widget

[artislismanis]

Fixes Vimux/Mainroad#341:

• Remove twitter.svg icon and add x.svg to be used instead. The X.svg adapted from Wikimedia Commons
• Update social widget to use the new title, website link and icon. Add support for both x and twitter as configuration keys - this ensures the existing configurations will continue to work where theme is included and updated as git submodule.
• Update theme documentation to show how to enable X in social widget.