fix#11
The activity form allow to receive pre-configured activity durations.
Those duration can be configured through the option
chill_activity.form.time_duration :
```yaml
chill_activity:
form:
time_duration:
- { label: '12 minutes', seconds: 720 }
# - ...
```
If a pre-existing activity receives a different time, the time is added
to the list of pre-configured duration time.
