Various improvements for 'Pending scans' view (now 'Pending changes').

- 'Device' column removed from 'New devices' tab, 'Venture' column added to 'Changed devices' tab;
- added fields for filtering the results by Venture and/or Service;
- added option to 'Show all items' in pagination (requires django-bob > 1.9.0 to work properly)
- expired scans are not showing in the results;
- changed 'pending scans' to 'pending changes'.