Update an existing Work Item Type

[tsmaeder]

Implement an API endpoint for updating a preexisting work item type.
An update may

• add a field
  This can be treated like in the case of a mixin: i.e. if the field is required, there must be a default given
• remove a field
  We could treat this as a hard delete and remove the values form existing work items or leave the values in the db, just not described by the type anymore
• change a field
  Im am not sure if we should allow this or rather insist on people creating new fields if they want to update them

Originally, I thought we could get by without updating types (just make new types), but if we created new base types, we would have to update all the derived types as well.
We should give some thought to the idea of versioned type...in that two work items can have types of the same name, but different work items.

The use case for updating work itemst is when we want to migrate between different versions of our system. I would expect that this will happen in the course of a planned migration. What the API should support is an upgrade path for an existing system: moving from usable state a to usable state b may require some migration tasks on the existing work items between the type updates.

[hectorj2f]

We're not going to support the update of work item type. We could support the creation of new types but not the update at this moment.