create spacy.app/delete-session command

[joyheron]

Once #25 has been implemented, we can add a delete-session command to spacy.app

This includes:

• creating a new route for our delete-session command
• creating a command-handler for delete-session analogously to the submit-session or schedule-session commands in spacy.app (parameters are probably: {:form {:id java.util.UUID}})
• Add this handler to the handler-map

To test this, you could generate a form in the UI that looks something like the following in either the queued sessions or the ones that are scheduled, but that is optional for this task:

<form action={{uris.spacy..app/delete-session}}>
    <input type="hidden" value={{item.session.id}}>
    <button>Delete Session</button>
</form>