Update addon to send data via postMessage rather than rely on GET params

[miketaylr]

This might not even be possible with existing web extension APIs... but Tom has some advice here:

mozilla/webcompat-team-okrs#34 (comment)

[miketaylr]

Here's a variation of what Tom is describing: https://stackoverflow.com/a/23687543/66348

[ksy36]

As we're adding postMessage data support on the webcompat.com side not only for screenshots, but for the rest of the metadata in webcompat/webcompat.com#2985 , lets send all data via postMessage to unify things.

So here https://github.com/webcompat/webcompat-reporter-extensions/blob/master/shared/base.js#L34 we'd need to send a postMessage with the following structure

        const data = {
          screenshot: blob,
          message: message
        };

where message should be a stringified object with url, src, utm_source and utm_campaign properties
and screenshot should be a blob instead of base64 string. This will also allow us to drop base64 support on webcompat.com side as described here webcompat/webcompat.com#1252