Edited document and its local resources must have the same origin

[justinfagnani]

Currently the edited document is modified (to insert the editing protocol handler and to label elements) before it's injected into the iframe via a blob URL. The blob URL has it's own origin, so loading resources from file:// URLs (as in Electron) is prohibited.

To fix this we can serve all files via a small server that does the modifications before the file reaches the browser.