Make it clear to the user when documents are being initialized

After WEBUI-134 we had to revert changes that cached documents during their initialization (on both the document create and import layouts) to ensure that server side events are triggered (eg. Empty Document Created). However, whenever the response the server takes a bit longer to return, the UI currently flickers between the document type selection and the appropriate document layout. If the network is congested and/or there's an event handler listening to the Empty Document Created, this is even more noticeable.  

Ideally we would like to display some sort of progress indicator between the time the user selects a document type and the time the appropriate document layout is displayed, to ensure that user understands that the UI is currently awaiting a response from the server.

This is also visible on 10.10, so depending on the selected approach we'll have to consider if it makes sense to backport to 10.10.