print current rust version on the about page

[QuietMisdreavus]

Closes #257

Now that we're using a real nightly compiler to build docs, let's show the version somewhere. This PR takes the suggestion from the linked issue and prints the version of the rustc inside the container on the "about" page.

Open question: Would it be better to cache this version string somewhere (say, in the database or in some lazy_static) so we don't have to attach to the container and run rustc --version every time someone loads the About page?

[QuietMisdreavus]

Output on my personal test server (which is using an older rustc than production, but the same principle should apply):

[dekellum]

LGTM, thanks!

[onur]

Like you said in your comment, problem with this approach is its running rustc (and lxc-attach etc.) every time when someone visits about page. I think its much better to cache rustc version. We can store it in our config table in database (it's a simple table with key and value members designed for this kind of things), and since we are running cratesfyi build add-essential-files every time when we update rustc, we can use this function to update value in database.

[QuietMisdreavus]

I've updated the PR to use the config table like you suggested. Loading it during add-essential-files is perfect. I've tested it on my dev server and it works great.

[onur]

We are using our r2d2 connection pool in our iron handlers. Instead of creating a new connection, getting req parameter and using let conn = extension!(req, Pool); should do the trick (similar to sitemap_handler).

[QuietMisdreavus]

Good catch! I've force-pushed an update with this change.