feat: add user-facing deployment cancel endpoint

Add POST /apps/{id}/deployments/cancel endpoint that allows users to
cancel in-progress deployments via the web UI.

Changes:
- Add CancelDeploy() and HasActiveDeploy() public methods to deploy service
- Add HandleCancelDeploy handler
- Wire route in routes.go
- Add cancel button to app detail template (shown during active deployments)
- Add handler tests for cancel endpoint

fixes #66

templates/app_detail.html

@@ -40,6 +40,10 @@
                     <span x-text="deploying ? 'Deploying...' : 'Deploy Now'"></span>
                 </button>
             </form>
+            <form method="POST" action="/apps/{{.App.ID}}/deployments/cancel" class="inline" x-show="deploying" x-cloak x-data="confirmAction('Cancel the current deployment?')" @submit="confirm($event)">
+                {{ .CSRFField }}
+                <button type="submit" class="btn-danger">Cancel Deploy</button>
+            </form>
         </div>
     </div>