refactor: remove proportion and salt params from Feature

[daniel-codecov]

The salt and proportion parameters in Feature are only ever used to create a new feature flag. If a feature flag with the same name already exists in the database, then the parameters are ignored.

This can be confusing as the params don't actually represent the values the feature flag is using 99% of the time. Its more safe to just always create feature flags with the default values (proportion=0, salt=randomlygeneratedstring) and we can modify them via django admin later.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.99%. Comparing base (a98be92) to head (9a982bb).

❗ Current head 9a982bb differs from pull request most recent head 27412c2. Consider uploading reports for the commit 27412c2 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #142      +/-   ##
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+ Coverage   90.93%   90.99%   +0.05%     
==========================================
  Files         116      115       -1     
  Lines        7083     7048      -35     
  Branches     1630     1623       -7     
==========================================
- Hits         6441     6413      -28     
+ Misses        615      608       -7     
  Partials       27       27              

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