WebExtensions: Installing an .xpi file should extract it
Currently we copy the file into our web_extensions directory and load the resources into memory. I want to move away from the model of keeping all resources in memory which this prevents. So I think we should just extract it and have a single codepath for resources that involves loading files. This will require some care to not do blocking IO in some places and maybe some resources should be carefully kept in memory. TBD.