Migrate from Twitter to X

[alancleary]

This will require the following changes:

• Update the icon in the navbar
• Update the caption of the icon in the navbar
• Add an x_username config variable and make the twitter_username variable an alias for it
• Add an x_card_item_limit config variable and make the twitter_card_item_limit variable an alias for it
• Add an x_card front matter variable and make the twitter_card variable an alias for it