Bug 886446 - [email] Partition card loading, cache initial card HTML for fast startup

Part of the front-end change to partition cards and use an HTML cache for fast startup.

For mailapi.js: with the HTML cache, the "_fake" data on the MailAPI is no longer needed.

For main-frame-setup.js: Removed a timeout in the wiring up of the worker, since now the HTML cache allows us to do code setup as it makes sense, without having to game startup times.

For copy-to-gaia.js: all the scripts in the email app's index.html are loaded via JS module APIs, so no need to modify the HTML source any more.