Add support for Focus releases

[Natim]

@sylvestre, @pocmo and @JohanLorenzo are in charge of Focus releases.

On IRC @pocmo told me that you might be interested in having a dashboard to follow Focus releases.
Any idea of what kind of checks you might want there?

[sylvestre]

Parsing https://play.google.com/store/apps/details?id=org.mozilla.focus (or an api if any) to know which version is live ?

[Natim]

Refs https://stackoverflow.com/a/25201516/186202

[JohanLorenzo]

There's an API, but it doesn't solve this particular problem.

The API we can use is the one used by mozapkpublisher: https://developers.google.com/android-publisher/api-ref/. It requires authentication (to be checked: see if we can get read-only credentials). From it, we may extract:

• What APK version code is currently up in any track: https://developers.google.com/android-publisher/api-ref/edits/tracks. This particular endpoint is more precise than parsing https://play.google.com/store/apps/details?id=org.mozilla.focus. There's usually a delay between the moment we pushed an APK, and the time its visible by all Google Play users.
• What recent changes are displayed to end users: https://developers.google.com/android-publisher/api-ref/edits/apklistings

And that's about it 😞

Note that APK version code aren't version numbers like "1.2.1". They are just integers that are mechanically increased. We can't get version numbers from the aforementioned API endpoint :/

Thus, I don't think this API has a lot of value from a product management perspective. So, I think parsing the Google Play page is our best alternative (even though lags may occur)