Create a custom forum badge for Alpha Testers

As part of the recruitment campaign being planned in tpo/applications/team#6, @gus has proposed creating a custom forum badge for Alpha testers. We'll need to figure out exactly how it will be awarded in #40075 (closed), but that shouldn't hold us up adding the badge itself to Discourse.

To do so, we need to:

1. Give the badge a name (e.g. Tor Browser Alpha Tester)
2. Select a font awesome icon or upload a custom one
3. Select a bronze, silver or gold level (or create one of each), which determines the badge's color
4. Add a short and long description
5. Choose from some other random prefs and save the badge