bug 1052453 - add waffled GitHub beta notice

Spot-check:
* [ ] Make sure you have a "Beta Testers" group via [the local django groups admin interface](https://developer-local.allizom.org/admin/auth/group/)
* [ ] [Add a `github_beta_notice` waffle flag via local django admin interface](https://developer-local.allizom.org/admin/waffle/flag/add/), and pick the "Beta Testers" group
* [ ] [Edit your local profile](https://developer-local.allizom.org/en-US/profile/edit) and check "Beta tester"
* [ ] You should see the global notice about the GitHub beta across the site now.
I would suggest we consider changing the second sentence to <i>Help us test GitHub login on our stage server</i>, and making the text <i>test GitHub login</i> a link rather than printing the URL directly. That would be a more clear call to action, and might do a better job of capturing the attention of users who skim the notice.