Replace Twitter with X in social widget

[artislismanis]

Fixes #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.

[mschwager]

Hey @Vimux, is it possible to merge this? I'm a consumer of the Mainroad theme, and we're hoping to update our socials references from Twitter -> X.