Handle last_modified only in FileLoader

[davidbrochart]

Fixes #230.

• last_modified is only handled in FileLoader, not in the rooms.
• File observers now only notify about out-of-band changes (rooms don't care about changes they are responsible for). As such, there is no argument to pass to callbacks.
• load_content effectively loads the content, thus the content parameter has been removed.
• save_content has been removed since it is not used.
• The shielded file save operation must be awaited even if the task that awaits it is cancelled, otherwise the file may be saved and last_modified not updated.
• notify has been renamed to maybe_notify, as it only notifies about out-of-band changes (it was previously notifying every second even if the file didn't change).
• A file save operation must be awaited if cancelled.

[github-actions]

👈 Launch a Binder on branch davidbrochart/jupyter_collaboration/fix-saving