Add beta progress to /builds.

Adds beta cycle progress meter to /builds#/beta.

Before:
![screenshot](https://www.monosnap.com/image/X6Y63PtuMZgV3QFlJZPke0wGW.png)

After:
![screenshot](https://www.monosnap.com/image/k9yUpiPMMAEbXBZeg8QN8pWCA.png)