Cyberstorm API: annotate has_changelog on PackageDetailAPIView

[anttimaki]

Refs TS-2006

[codecov[bot]]

Codecov Report

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

Comparison is base (d16d50b) 92.93% compared to head (49778d7) 92.93%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #969 +/- ## ========================================== - Coverage 92.93% 92.93% -0.01% ========================================== Files 286 286 Lines 8507 8505 -2 Branches 790 790 ========================================== - Hits 7906 7904 -2 Misses 497 497 Partials 104 104 ```

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[anttimaki]

@MythicManiac ready for another review.

• Empty strings are now considered valid changelogs
• Added a second commit that returns 404 if changelog is None