[feat] Add support for calling bitmask.js from Firefox/Thunderbird

To be able to call bitmask.js from Thunderbird (for the Bitmask Thunderbird extension), we need to set 'api_url' and also read the 'authtoken' file from disk. This commit adds support for that and restricts the changes by using window.location.protocol.

I have verified this by calling it from bitmask_thunderbird, changes which I will be pushing later.