create spacy.app/edit-session command

[joyheron]

Once #31 has been implemented, we can add an edit-session command to spacy.app

This includes:

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

It's still a little unclear how the UI will look for this. Theoretically we could link to a /{event-slug}/edit-session?id={id} resource from within our session which would return the form with which we could submit our edit-session command.