Create TinyMCE Draft save/restore integration

In Atto this is handled as a plugin. We should see whether that is appropriate in this case.

There is an existing TinyMCE autosave plugin, but this does not upload to the server (it uses LocalStorage) and is therefore unsuited to our requirements.

We should be able to base what we do on that plugiun to some extent, but replacing the LocalStorage with remote storage and changing the back-off to not negatively affect server performance.