Simplify case-activity forms and add case_status as a url param

The specific requirement for CIviHR is to have a clean, simple form for changing case status. Currently the "change case status" form has dozens of elements on it which are really distracting.
The other requirement is to be able to pass case-status-id as a url param so that it can be pre-selected.
IMO the best solution for the first requirement would be a general cleanup of the change_case_* activity forms in core CiviCase (change case status, change case type, change case start date, etc). I think this could be done fairly simply with a tpl tweak. The result would be that upon opening the form the user would see only 2 fields:

• The select field for case_[status,start_date,type]
• The rich-text "description" field for them to type in any notes.
  All the other fieds would still be present but would be moved into a collapsed fieldset.
  The 2nd requirement should be a simple matter of reading (and validating) url arguments, setting default values and freezing the form element. Although our immediate requirement is only for case status, doing this in a reusable way that applies to all form elements would be really nice.