[Website] Fix Twitter button and add LinkedIn button

[ianmcook]

This fixes the broken Twitter follow button at the top of the Arrow homepage, replacing it with an X follow button. This also adds a LinkedIn follow button.

I styled these new buttons to have the same appearance as the GitHub star social buton. Unfortunately there is no clean way to achieve this. I had to inspect the page, grab some client-side-generated CSS, modify it, minify it, and include it in the HTML to style the new buttons. Not a clean fix but I think it's worth it to have the prominent broken Twitter link finally fixed.

Before:

After:

[ianmcook]

I took another look at this today to see if it would have been possible to achieve this more cleanly with Shields.io, but it turns out no, because they depend on Simple Icons which recently removed their LinkedIn icon based on an interpretation of the terms of use.

Our use of the LinkedIn logo is in compliance with the LinkedIn brand policies (1, 2) per my interpretation, although these policies contradict themselves. Quoting from the relevant sections:

if you are a LinkedIn member, you may use the “in” Logo to designate a link to your personal profile or to a Group or Company page or to promote you or your organization’s presence on LinkedIn.

While our default logo is blue, use the black or white version on layouts that are black and white only. You can also use the white version on dark backgrounds so it's easy to see.