consider allowing users to specify beta numbers in Rules

[bhearsum]

Modern Firefox Betas (5.0+) do not send their "Beta" number (eg: 52.0b3) as part of update requests. Instead, we distinguish Betas by versions+buildid. This can cause some confusion, as Beta numbers are standard parlance (eg: bug 1479579).

It would be great if Balrog would support configuring Rules with Beta #s. I'm not sure what the best way to do this would be, but here's a couple of ideas:

• Allow users to specify things like "52.0b3" in the UI, and have it translate it to a buildid by looking at some other source of truth (Ship It?). It would then fill in version+buildid properly, and put the human readable version in the comment field.
• Allow things like "52.0b3" in the "version" field, and have the backend do the translation at update request time. This would mean the backend would either need to look up (and cache) beta #s -> buildids on demand, periodically sync them from somewhere, or otherwise maintain a mapping. There's a risk here that Balrog Rules could now be influenced by an outside system that we'd have to be willing to take.

(Imported from https://bugzilla.mozilla.org/show_bug.cgi?id=1481521)