[Retro] Add configurable timer to brainstorm phase

[StevenWeathers]

Most teams likely schedule a timeslot for Retro and having the ability to timebox brainstorming phase would be helpful.

[scottux]

ok, so

models

retro creation would get a timebox option, which would be a selectable number of minutes between 1 and 10 per phase.

phases would have a startTime timestamp attribute

logic

if phase's parent _retro timebox minutes_ > 0
  on phase start
    set the _startTime_ timestamp attribute
    countdown from _startTime_ for _retro timebox minutes_

if _now_ > (_startTime_ + _retro timebox minutes_)
  set phase done

optionally

we could also add a timerFinal boolean option to control the logic for setting phase done or not.