Use AMO API to fetch latest xpi

[Rob--W]

The XPI URL is currently based on https://github.com/Rob--W/crxviewer/blob/2972e4fe9f15b703f07f157fc5d3fa2e2fe97b43/src/cws_pattern.js#L60. This returns the URL for the latest reviewed XPI file.

For reviewers, the shown XPI should preferably be the URL of the latest (potentially unreviewed) add-on. It seems that the only way to do this is through the API: http://addons-server.readthedocs.io/en/latest/topics/api/index.html / https://addons-server.readthedocs.io/en/latest/topics/api/addons.html#detail .