Custom break time per session

[scottgonzalez]

trafficstars

Sessions should allow an override for how long the break before clean and jerk should be. This will allow adjusting the break at the start of the session based on schedule adjustments.

If the field is not filled in, the existing automatic clock will be used (if enabled).

There should also be a getter for the computed value on the session instance so that it can be used in templates.