When a resource is shared to MoodleNet as an activity backup, that backup will be created and occupy space on the server. We should perform relevant cleanup operations to ensure these are not stored indefinitely, either after the file has successfully be uploaded to MoodleNet, or after failure to send (since we currently do not have a way to retry the file again later).

Two of the possible implementations are either as a scheduled task to perform the cleanup, OR on-demand, so the sharing API cleans up as it completes its task (which may be easier since it will only have to keep track of the file it has created, whereas a scheduled task may not be able to easily distinguish which files are no longer needed).