Add last coffee pack button

Add last coffee pack button

When clicked, the message notifying Tonda about the lack of coffee is
sent to the Slack.

There must be `.config` file in `coffee-flask` folder. The `.config`
file is JSON formated. For this patch the following is needed:
```
{
        "coffeebot": {
                "url": "https://hooks.slack.com/services/..."
        }
}
```