This puts the app into its own origin so it doesn't have access
to the filesystem via file:// URIs.
Next step: migrate over localstorage & indexeddb data from the old
origin...
* Turn off node integration in the electron renderer process
* Enable the chromium sandbox to put the renderer into its own process
* Expose just the ipc module with a preload script
* Introduce a little IPC call wrapper so we can call into the
renderer process and await on the result.
* Use this in a bunch of places we previously used direct calls
to electron modules.
* Convert other uses of node, eg. use of process to derive the
platform (just look at the user agent)
* Strip out the desktopCapturer integration which doesn't appear
to have ever worked (probably best to just wait until
getDisplayMedia() is available in chrome at this point:
https://github.com/vector-im/riot-web/issues/4880).
David Baker