Deposit file selection improvements

[tomhigginsuom]

The deposit UI could be improved. For example, the current tree view can make it difficult to determine what has been selected to deposit (because it requires the user to click a radio button). A simpler file browser such as the GitHub file interface might be clearer and more usable.

In addition the system currently doesn't allow a deposit if nothing is selected (e.g. the top-level - an error message is displayed because the broker API doesn't know which file system the user wishes to use). What should the behaviour be if no file/directory or file system is chosen? A more informative message (perhaps confirming what is about to be deposited) might be useful.