Extend our build scripts to allow betas, release-candidates

We should update the current setup with a build beta / release candidate deploy.

- The normal Beiwe app does to a link accessible by the download link on the bottom right corner of a Beiwe website, which is at `/download`
- We can make a new secret link, `/download-beta` that testers can just type into their browser.
- Would need to clear the beta out whenever we do a non-beta build.