Expand model to consider resource allocation / leave calendars / etc

The model currently assumes that tomorrow's velocity will be the same as yesterday's. It _might_ be useful to teach it to understand common reasons why this might not be true:
* team size changes (new team member, existing team member leaving)
* resource allocation changes (existing team member moving from 100% allocation to 50%, or vice versa)
* upcoming (predictable) leave (annual, public holidays, etc)

You could start with allowing the user to specify this stuff manually, and then build on it by tapping into leave calendars and the like.