Add X link and icon to header

[kuzdogan]

I think we should have an X link that is more visible. While on it I've styled it a bit

[abcoathup]

Great idea @kuzdogan
When I tested the PR locally, the GitHub & X icons didn't appear (tried system/light/dark modes).
When changing the browser size to mobile, text for GitHub & X menu items wasn't shown.

[kuzdogan]

I was able to reproduce it. It should be fixed now can you check again?

[abcoathup]

@kuzdogan when resizing for mobile:

• hamburger menu shows icons for GitHub, Twitter and home, this should either be text, or appear next to dark/light icon at top of menu.
• | is a link to home and appears in the hamburger menu.
• GitHub & X icons don't hide for mobile, but the dark/light mode does

[kuzdogan]

Thanks, done