Add remote layout management to UI

[pling-scottlogic]

Background

In #20, we create a front-end interface to handle all layout persistence. We need to make an implementation of this interface to communicate with the REST service created in #25 to handle remote layout storage.

Base on #20 branch

Work Required

• add environment variable to pick local or remote
• Make a "remote storage" implementation of the layout persistence interface
• Have the load/save/update/delete methods call through to the appropriate GET/POST/PUT/DELETE endpoints
• Handle potential HTTP 500 errors (and any other non-2xx) appropriately

Testing

We will need to mock the API response.