Render the commit hash on the page

[Zegnat]

Not sure if this is a good idea, but it would allow us to check which commit from the repository is actually being used by the website. Gives a quick sanity check on the state of the Heroku build.

This uses an environment variable inserted by Heroku:

If you need to determine the currently deployed revision of your app, either use the env var SOURCE_COMMIT during the build, or enable the Dyno Metadata feature and use the env var HEROKU_BUILD_COMMIT at run time.

@aaronpk if I have understood the Heroku docs correctly, this needs some flag to be enabled by you?

heroku labs:enable runtime-dyno-metadata -a <app name>
heroku labs:enable runtime-dyno-build-metadata -a <app name>