create spacy.app/move-session command

[joyheron]

Once #26 has been implemented, we can add a move-session command to spacy.app

This includes:

• creating a new route for our move-session command
• creating a command-handler for move-session analogously to the submit-session or schedule-session commands in spacy.app (parameters are probably the same as with schedule-session)
• Add this handler to the handler-map

Note:

For the UI later this will probably be implemented by creating a new /{event-slug}/move-session endpoint which will render a very similar table to the one in the event.html page but where the actions post to the new move-session command instead of to schedule-session. If you want, you can attempt to implement this. If not, that can be implemented in a separate step.