Expiry dates are reset when item state changes

Moved expiry code from Feed._update_entries into EntryQuerySet.set_expiry
Added EntryQuerySet.clear_expiry
Added calls to clear_expiry from views which unset state=ENTRY_READ

This should be the second part to the fix in 0.4.0 to stop unread count falling
out of sync
Changed scope of constants
Tidied button creation
Tidied dev notes
Fixed dropdowns