Cyberstorm API: minor improvements to BasePackageListAPIView

[anttimaki]

• remove unnecessary definition of get method
• in filter_queryset, use the queryset passed as a parameter rather than refetch in unnecessarily
• in filter_queryset, avoid overriding the queryset received as a parameter, although DRF's implementation actually does that

Refs TS-1875

[anttimaki]

Current dependencies on/for this PR:

• master
  • PR #907
  • PR #908 👈
    • PR #909

This stack of pull requests is managed by Graphite.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (d1a524a) 92.48% compared to head (10a8e20) 92.48%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #908 +/- ## ========================================== - Coverage 92.48% 92.48% -0.01% ========================================== Files 274 274 Lines 8037 8034 -3 Branches 763 763 ========================================== - Hits 7433 7430 -3 Misses 501 501 Partials 103 103 ```

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