Tighten up the wording on the Twitter link.
Add Twitter icon to the home page, linking to @official_php.

I've deliberately implemented this in such a way as to allow for further social
media buttons down the track if we want. I've also kept it intentionally
unobtrusive: it's there (previously, we only mentioned the Twitter account on
the mailing lists page, which was weird), but pretty subtle in the sidebar.

There's one additional dependency: a one glyph subset of Font Awesome (created
using Fontello) containing the Twitter icon. (Should we later want to include
other icons from Font Awesome, fonts/Font-Awesome/config.json can be fed back
into Fontello as a starting point.) The additional style sheet and font file
are cached correctly, and once loaded by the user, shouldn't need to be loaded
again, as with Fira Sans. On a cold page load, this adds two requests and just
under 3 kB to the page load.

I consider all of this reasonable, both technically and in terms of promoting
@official_php a little more, but I won't be terribly offended if someone wants
to revert this.