Enable deletion of assignments. Issue #33.

Only for !period.keep_updated? periods.
TODO: Enable for updated periods as well?
Enable editing of assignment.status for non-updated periods. Issue #33.
Merge branch 'master' into delete_assignments

Need to use @list_type that gets set in upstream master branch.
Enable display of all assignments for a period - including inactive ones.

Also add a display_all checkbox to list/_form that selects all/none of
the report fields.