Fully support "start on demand"

[dorianim]

There needs to be an option to start the timer on deband with the push of a button. This is done by adding a "placeholder" segment in the end, which only lasts one second and counts down to the desired time.
When the timer is done, that segment is shown and once the timer is restarted, it will look like it just starts.

• show end of last segment when timer is done
• add an option to put the start time a few seconds into the future when anything in the timer is changed This will make sure, that all displays have received the update and start counting synchronized (Chore: capsulate redis operations into respository module #26)
  • This will be done by adding a metadata struct to the timer which contains an integer field delay_start_stop, which will be used as the delay after pressing restart.
  • The metadata struct shall not be present on the websocket
• add an option ShowLastSegment to the pre_start_behavior (Chore: capsulate redis operations into respository module #26)
• make the sounds more configurable, so a start sound can be added
  • The sounds will be at /sounds/<sound file name>
  • each segment will have a sounds array which contains structs with the properties filename and trigger_time
  • maybe check if trigger_time is contained in the segment
  • trigger_time is given in seconds and means the beginning of the given second.