Fix unsetting assignee and milestone.

The bindings expect user and milestone to be set; if they're null it
means 'don't touch it'.
Use the correct selected background on API 19.
Fix commit file item padding and use a consistent style for header
labels.
Overhaul issue and milestone activities.

Provide a little more padding (and make the padding consistent in
general) and fix code duplication between milestone creation and
editing.
Allow unsetting a due date.
Fix cancel behaviour of milestone/assignee selection dialogs.