Chat: Add load handling for 'All files' and 'All channels'

Motivation

Currently there is no visual indication that the files and channels are loaded. If the internet connection is slow, there's also a bug where "There are no files/channels yet" is shown when the request is not even finished yet.

Implementation notes

• use options object with success and fail handler in collection.fetch()s in the views.
• use native .busy() function to show a loading spinner
• use native .idle() function to hide loading spinner on finish