Improve export cancellation

[bryfox]

Fetch requests in the AdminAPI should be made cancellable on the client side by an AbortController, supported as of Chrome 66 / Electron 3.

Note that polyfills like https://github.com/mo/abortcontroller-polyfill won't actually abort the request; work proceeds happily on the Server after abort is called. Currently, the workaround is to reload the page. Obviously, this workaround should be removed.