Add a way to specify the kinto server to use during form creation.

[almet]

Users should be able to specify which kinto server they want to use during the creation of the form (e.g. it shouldn't only be a configuration of the application itself).

I can foresee a world where it would also be possible to not only send the schema to the server, but also the complete application that consumes the schema (using kinto-attachments), and then maybe another plugin that would return the index.html page if the collection is accessed via a browser (By checking the Content-Type header + the presence of an attached file).

This way, form consumers could just have to converse with the storage server to store data there.

[n1k0]

We could also provide an input field for for admins to provide the URL of the Kinto server they want the data to be published to. This Kinto server URL could be passed in the app url, so no need for any configuration.