File status (beta or not) is explicit on upload (bug 669674)

Fixes [bug 669674](https://bugzilla.mozilla.org/show_bug.cgi?id=669674)

The rationale here is to allow a developer to explicitely specify the beta
status of a file uploaded.

If a file has a version that looks like beta, the "beta" checkbox is
pre-selected.

![beta_version](https://cloud.githubusercontent.com/assets/167767/4573374/b06ff6f4-4f91-11e4-9e83-a7f2b03dc6ba.png)