Clock out from slack

As a user, I want to be able to clock out from slack in order to have another alternative if I am out of the office.

Acceptance Criteria
- Add a new command in slack that calls the /clock-out endpoint. A suggested way would be to type the following command in slack:

```
/time-tracker clock-out
```
- After executing this command, the clock-out will be invoked and the entry running will be closed.